Classification

by Birger Hjørland

Table of contents:
1. Introduction
2. The meaning of the word classification
3. Related terms
    3.1. Concept/conceptualization
    3.2 Categorization
    3.3 Ordering
    3.4 Taxonomy
    3.5 Typology
    3.6 Division
4. Theories of classification
    4.1 The "classical view" versus "prototype theory"
    4.2 The methodology of forming classes
        4.2a Elaine Svenonius: α Operationalism, β The picture theory of meaning, γ The contextual
theory of meaning
        4.2b Alberto Marradi: α Subdivision, β Numerical taxonomy
        4.2c Birger Hjørland: α Rationalism, β Empiricism, γ Historicism, δ Pragmatism
    4.3 Some metaphysical issues of classification: is there one correct classification?
        4.3a Artificial versus natural classification
        4.3b Order versus disorder of things
5. Conclusion
Acknowledgments
Endnotes
References
Appendix: A sample of definitions of classification (chronological)

Abstract:
This article presents and discusses definitions of the term classification and the related
concepts concept, categorization, ordering, taxonomy and typology. It further presents and
discusses theories of classification including the influences of Aristotle and Wittgenstein. It
presents different views on forming classes, including logical division, numerical taxonomy,
historical classification, hermeneutical and pragmatic/critical views. Finally, issues related to
artificial versus natural classification and taxonomic monism versus taxonomic pluralism are
briefly presented and discussed.

1. Introduction

This article is about classification as a basic term in an interdisciplinary perspective.
Classification is a fundamental concept and activity in knowledge organization, but it is also an
important concept in many other fields, including biology and philosophy. In knowledge organization
and library and information science (LIS), it is mostly about classifying documents, document
representations, and concepts (e.g., in thesauri), and library classification systems and
ontologies are well-known kinds of knowledge organization systems (KOS). These activities and
systems are based on more fundamental conceptions and theories of classifications that are
presented in this article.

The ISKO Encyclopedia of Knowledge Organization (IEKO) plans to cover a very broad spectrum of
articles related to classification besides the present one. We already have an article about
→ logical division, → notation, → automatic subject indexing, and further articles are planned
about, for example, library classification, numerical taxonomy, classification of the sciences,
classification in specific domains (including biology, physics and chemistry), and much more.

This article covers a very complex concept and is therefore highly compressed and abbreviated. In
particular, most of the theories mentioned in section 4 deserve to be enlarged on in independent
articles that it is hoped will be forthcoming at some point in the future.

2. The meaning of the word classification

Among the many contributors to the definition of classification two (Frederik Suppe and Henry E.
Bliss) are here selected as outstanding. Frederick Suppe distinguished two senses
of classification: a broad and a narrow meaning. He called the broad meaning "conceptual
classification".

Classification is intrinsic to the use of language, hence to most if not all communication.
Whenever we use nominative phrases we are classifying the designated subject as being importantly
similar to other entities bearing the same designation; that is, we classify them together.
Similarly the use of predicative phrases classifies actions or properties as being of a particular
kind. We call this conceptual classification, since it refers to the classification involved in
conceptualizing our experiences and surroundings" (Suppe 1989, 292).

Classification in the narrower meaning Suppe called "systematic classification".

A second, narrower sense of classification is the systematic classification involved in the design
and utilization of taxonomic schemes such as the biological classification of animals and plants by
genus and species (Suppe 1989, 292).

Henry E. Bliss (1929) also considered the senses of the word classification and wrote: "[t]his
term, like other English derivatives ending in ion, is ambiguously used both in the predicative and
in the substantive sense, now for the action and now for the act, sometimes for the process and
sometimes for the product" (142). In order to remove this ambiguity, he suggested three definitions
proceeding from class as a substantive:
 1. The verb to class denotes likening, referring, or assigning a thing to some class, or several
things to their respective classes, as may be requisite or relevant to interest involved. This verb
is used not only transitively, but sometimes intransitively. Thus it may be said that olive
oil classes as a luxury.
 2. The verb classify means primarily to make, or conceive, a class, or classes, from a plurality
of things, and secondary to arrange classes in some order or to relate them in some system
according to some principle or conception, purpose or interest [...]. These three processes,
classing, forming classes, and arranging classes, are so implicated that it is not easy to separate
them in thought or in terminology; yet we propose here that this should be done as conductive to
precision in this study. There is an important distinction between assigning a thing, or things, to
some class or classes, and arranging classes in some order or system. [...]
 3. A classification is a series or system of classes arranged in some order according to some
principles or conception, purpose or interest, or some combination of such (Bliss 1929, 142-143).

There are many more definitions of classification than the ones given above. For a chronological
sample of definitions of classification, see the Appendix.

The objects we classify may be physical objects, persons, processes, ideas, concepts, words, etc.
Some of these entities, such as concepts, may be both the elements classified and a result of a
(new) classification.

         Table 1: Selected terms used about the units classified and the resulting groups

What may be classified

                                                              Results of classification may be termed:

Concepts

                                 Categories

Documents

                                 Clades

Elements

                                 Classes

Entities

                                 Concepts

Ideas (including fictional ideas)

                                 Genera

Individuals

                                 Groups

Items

                                 Kinds

Objects

                                 Sets

Phenomena

                                 Sorts

Processes

                                 Species

Sciences

                                 Taxa

Things

                                 Etc.

Etc.

The objects to be classified have attributes with values. Attributes may, for example, be color or
weight. Values may be red or heavy. Classifications are made by considering different attributes
and their values. [1]

In conclusion: Classification is a term used both about the process to classify (which is a kind of
discriminative practice; see Schmidt and Wagner 2004, 392) and about the resulting set of classes,
as well as the assignment of elements to pre-established classes. The wide meaning
of classification is the process of distinguishing and distribution kinds of "things" into
different groups. All narrower meanings of classification are based on the wide definition but add
some extra requirements or restrictions put to the classification process and the resulting
classification system — for example, the requirement that a classification should use only one
criterion of division at a time, that classes should be mutually exclusive, and jointly exhaustive,
are requirements demanded by some specific theories of classification, but not requirements that
are common for all kinds of classification as here defined.

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3. Related terms

3.1 Concept/conceptualization

Concept has formerly been defined the following way:

Concepts are dynamically constructed and collectively negotiated meanings that classify the world
according to interests and theories. Concepts and their development cannot be understood in
isolation from the interests and theories that motivated their construction, and, in general, we
should expect competing conceptions and concepts to be at play in all domains at all times"
(Hjørland 2009, 1522-1523).

There is a close relationship — if not total identity — between theories of classification and
theories of concept. The class of waterfowl, for example, includes the subclasses ducks, geese,
and swans, in exactly the same way that the concept "waterfowl" includes the subordinate concepts
of "ducks", "geese", and "swans". The different theories of how we classify birds correspond to the
theories of how we conceptualize birds (see Andersen et al. 2006, 19-33). Henry Bliss also
emphasized this:

It is evident that a discussion of classes involves the correlation of classes to concepts, or
class-concepts. The class-concept is the mental correlate of the class, the mental basis both of
the general idea of the class and of its name, or names (Bliss 1929, 120; italics in original).

Spiteri (2008) found that an examination of traditional similarity-based concept theories suggests
that they do not provide an adequate account of conceptual coherence. Library and information
science needs to explore knowledge-based approaches to concept formation, which suggest that one's
knowledge of a concept includes not just a representation of its features, but also an explicit
representation of the causal mechanisms that people believe link those features to form a coherent
whole. Spiteri (2008) found that rather than representing a universal truth based upon unitary
descriptions of concepts, classification systems represent only particular points of view. She
hereby supports the argument of Hjørland and Albrechtsen (1999) and Beghtol (2003) that
classification research must be situated within specific contexts and the domains in which the
classification systems are designed to function, as well as Mai's (2004, 41) claim that "Any
classification is relative in the sense that no classification can be argued to be a representation
of the true structure of knowledge [...] a classification is merely one particular explanation of
the relationships in a given field that satisfies a group of people at a certain point in time".

Frické (2012, 33), however, is opposed to considering concepts as mental constructs, and writes
that the word concepts "amounts roughly to "general notion" or "general idea" or even "meaning".
Many describe concepts as being mental or mental constructions; however, we regard them as
abstractions or abstract objects (in the standard Fregean third realm)."

In spite of this close connection between classification and concepts, the discourses on concepts
and the discourses about classification seem mostly to be separated in the literature.

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3.2 Categorization

Elin K. Jacob found that classification and categorization are different processes:

Although systems of classification and categorization are both mechanisms for establishing order
through the grouping of related phenomena, fundamental differences between them influence how that
order is effected — differences that do make a difference in the information contexts established
by each of these systems. While traditional classification is rigorous in that it mandates that an
entity either is or is not a member of a particular class, the process of categorization is
flexible and creative and draws nonbinding associations between entities — associations that are
based not on a set of predetermined principles but on the simple recognition of similarities that
exist across a set of entities. Classification divides a universe of entities into an arbitrary
system of mutually exclusive and nonoverlapping classes that are arranged within the conceptual
context established by a set of established principles. The fact that neither the context nor the
composition of these classes varies is the basis for the stability of reference provided by a
system of classification. In contrast, categorization divides the world of experience into groups
or categories whose members bear some immediate similarity within a given context. That this
context may vary — and with it the composition of the category — is the basis for both the
flexibility and the power of cognitive categorization" (Jacob 2004, 527-528).

Jacob's distinction is based on the narrow meaning of classification presented in section 2. Her
distinction is supported by Schmidt and Wagner (2004), who introduced some distinctions between
classification and other forms of discriminatory practice:

The point we want to make is that we have to be quite specific in distinguishing different types of
discriminative practice: seeing something, seeing something for what it is as opposed to something
else (reflecting on what one is seeing), physically separating things in some regular way, saying
that x is C ("categorizing" x as C), and classifying x as C according to an inscribed, publicly
available classification system. These are radically different practices, involving radically
different forms of convention, principles of abstraction, etc. (Schmidt and Wagner 2004, 45-46).

About categorization, the same authors wrote:

Categorization, by contrast [to seeing and recognizing], is a linguistic operation of ascribing a
category or concept to a particular phenomenon by the means of signs. Merely talking about
phenomena, however, is not necessarily categorizing them, although talking involves the application
of concepts. To categorize is to make a conceptual proposition ("red is a color").
In categorizing what you see as trees and birds you emphasize certain aspects of the world while
abstracting from others, for instance that the trees and birds may all have green colors or that
clouds and leaves may all be moved by the wind. An act of categorization cuts the world into pieces
in that it emphasizes certain features at the expense of others ("x belongs to category C").
(In themselves acts of separating objects are not acts of categorization, as they are not
necessarily linguistic operations. Peeling onions or removing dirt from one's body by means of soap
and water are acts of separation but not acts of categorization, although they may be subjected to
acts of categorization, for instance when one is instructing children in how to do it. Similarly,
when sorting the garbage (putting paper in this container, potato peels in that container) one may,
or may not, be following instructions involving categorizations)" (Schmidt and Wagner 2004,
391-392).

These two sources pose a question about the broad definition of classification mentioned in section
2 of the present article. Nevertheless, this broad definition is widely used in the literature and
it will introduce problems to restrict the term classification to the narrow definition. Therefore,
the choice made here is to consider classification as synonymous with categorization but to
maintain the distinction between classification in a wide and a narrow sense.

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3.3 Ordering

WordNet 3.1 provides two senses of the noun ordering:

·         ordering, order, ordination (logical or comprehensible arrangement of separate elements):
"we shall consider these questions in the inverse order of their presentation"

·         order, ordering (the act of putting things in a sequential arrangement): "there were
mistakes in the ordering of items on the list"

Some authors do not consider historicist classifications (like cladistics systems) as following the
concept of classifications:

For several centuries all ordering systems were thought to be classifications and the two terms
were treated virtually as synonyms. Eventually, however, it was realized that classification means
making classes and that ordering systems that are not based on classes, such as sequential listing
or cladifications (Mayr 1995), are not classifications. Hence, ordering systems denotes the general
concept that includes classification as one of its subdivisions. (Mayr and Bock 2002, 172)

Instead, Mayr and Bock suggest that cladistics systems should be considered as ordering systems in
a broader category. However, this terminology is not generally used, and it is deviant from the
suggestions made in the present article.

Ordering depends on conceptual classification but it is broader than systematic classification.
Books can be ordered by, for example, by size, language, or publication date, or alphabetically by
author or title.

For further information about order and ordering systems, see Meinhardt et al. (1984), Mayr (1995),
Mayr and Bock (2002) and Schmidt and Wagner (2004).

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3.4 Taxonomy

The term taxonomy originates from Greek taxis (meaning "order" or "arrangement") and nomos (meaning
"law" or "science"). It was first used in 1813 by the French naturalist Augustin-Pyramus de
Candolle (Candolle 1813). It became widely used in relation to biological classification but has
since spread to other domains. Some authors consider it synonymous with classification, whereas
others make distinctions between the two terms. In the following quote, the two terms are
considered synonymous:

[T]axonomy has acquired a wide range of meanings no longer restricted to the classical
understanding of biology. Taxonomy is now applying its early sense of organizing things in accord
with particular principles ("taxis": arrangement; "nomos": law) to a broader range of domains after
several centuries of limitation to biology and other natural sciences. In the 1990s, taxonomy was
redefined as any semantically significant, systematic organization of content or as the process of
developing such organization. This definition sometimes includes any collection whose individual
elements have been assigned to various nodes of a classification system. Thus, taxonomy is
sometimes considered the process of matching collection items with predefined labels, and sometimes
it is the creation and arrangement, as well as the resulting product, of the classification system
itself (Grove 2010, 5139; references omitted).

An example of a distinction between classification and taxonomy is:

'Taxonomy' is generally considered a synonym of 'systematic' and is traditionally divided into
classification (the orderly arrangement of organisms into taxonomic groups on the basis of
similarity), nomenclature (the labeling of the units), and identification (the process of
determining whether an unknown belongs to one of the units identified). (Vandamme 2015, 255).

Adams and Adams (1991, 202-3):

The term taxonomy, like other words relating to classification, has no generally accepted, precise
definition. For many systematists, especially in the biological sciences, it is synonymous with
classification itself [references here omitted]. Our usage, however, is more restricted. We apply
the term only to classificatory systems having an explicit hierarchic feature; that is, systems in
which basic types are either clustered into larger groups or split into smaller ones, or both.
Most of the time, taxonomic ordering is simply the classification of classes: the clustering of
basic types into larger and more inclusive units on the basis of some but not all of their
attributes […]
The classic example of a taxonomy is the Linnean system of biological classification, in which
species are clustered into genera, genera into families, and so on up to the level of biological
classes.

Hedden (2016) also uses the term taxonomy in a very broad sense, not just about classifications
(hierarchical or non-hierarchical) but as a synonym for any kind of knowledge organization system
(KOS). However, one may ask, if the term taxonomy is not used with a specific meaning in relation
to classification and KOS, why then use it at all? Hlava (2014) is another book that uses the word
"taxonomies" in the title, but does not distinguish between "taxonomy" and "classification". In the
glossary is written: "Taxonomy. As defined in ANSI/NISO Z39.19-2010R, 'A collection of controlled
vocabulary terms organized into a hierarchical structure. Each term in a taxonomy is in one or more
parent/child (broader/narrower) relationships to other terms in the taxonomy'" (Hlava 2014, Vol. 1,
50). This way of using terminology is confusing; instead of trying to clarify terms and to aim at
consensus in the use of terms for given concepts, it does the opposite.

ISO 25964-2: Information and Documentation-Thesauri and interoperability with other vocabularies,
Part 2: Interoperability with other vocabularies (International Organization for Standardization
2013) contains a lengthy section 19 "Taxonomies" (p. 59-66), in which taxonomies are just defined
(60) as "monohierarchical as well as polyhierachical classification of any subject matter" (i.e., a
hierarchical classification). However, the same standard writes in section 3.10 (3) "Classes are
the basic units of which a classification scheme is constructed. In a taxonomy, although this is a
type of classification scheme, they are generally known as categories". Aside from demanding that a
taxonomy is hierarchical, no defining characteristics are given in this standard.

Dahlberg's Classification System for the Literature on Knowledge Organization has class 5 "On
Special Objects Classification Systems (Taxonomies)", as opposed to class 6 "On Special Subjects
Classification Systems" (Dahlberg 1993, 219). Similarly, according to Aida Slavic the difference
between classifications and taxonomies is based on the distinction between aspect classification
(or "disciplinary classification") on one side and entity classification (or "phenomenon
classification") on the other.

Knowledge classification can be, and often is, TAXONOMIC (sometimes called "entity classification")
like the classification of zoology, classification of plants, or classification of chemical
elements (which means that they are going to list one concept in one place only in the
classification structure).
Bibliographic classifications i.e. those one has to use to describe real documents ARE NOT and CAN
NOT be taxonomic. They are by all means ASPECT or disciplinary classifications. This means that
they will list one concept in all disciplines and fields where that concept might be studied: e.g.
"water" will have to appear under chemistry, physics, in geology, medicine, sport etc.
This is of critical importance for information retrieval as aspect classification helps to
establish the context in which one concept or phenomenon might be studied within the document"
(Slavic 2000; electronic source, no pages).

This quote from Slavic is, however, contradicted by other uses of the terminology. Bibliographical
classifications may be phenomenon classifications (for example, the system by James Duff Brown
(1862-1914); cf. Beghtol 2004; Gnoli 2016) and disciplinary-based library classification systems
like the Dewey Decimal Classification (DDC) are also sometimes termed "taxonomies" (see Waltinger
et al. 2011).

Carl E. Landweh et al. found that a taxonomy is based on a theory.

A taxonomy is not simply a neutral structure for categorizing specimens. It implicitly embodies a
theory of the universe from which those specimens are drawn. It defines what data are to be
recorded and how like and unlike specimens are to be distinguished. In creating a taxonomy of
computer program security flaws, we are in this way creating a theory of such flaws, and if we seek
answers to particular questions from a collection of flaw instances, we must organize the taxonomy
accordingly (Landweh et al. 1994, 214).

However, classifications, too, are based on theories (and an atheoretical classification or
taxonomy may be considered an oxymoron; see Hjørland 2016b). Therefore, the theoretical basis
cannot be used as a criterion for distinguishing classification and taxonomy.

Marradi suggested the following distinctions:

A taxonomy obtains when several fundamenta divisionis [criteria of division] are considered in
succession, rather than simultaneously, by an intensional cl. [classification]. The order in which
fundamenta are considered is highly relevant: the taxonomy obtained by using property X to classify
a genus and then property Y to classify its species is by no means the same as that obtained by
considering property Y first and property X afterwards (Marradi 1990, 146).

Marradi suggested the following differences between classifications, typologies, and taxonomies as
products:

1.      Classification schemes
When only one fundamentum divisionis is considered, a classification scheme is produced — usually
by an intensional classification. The extensions of each class must be mutually exclusive, and
jointly exhaustive. Classes need not be at the same level of generality, and may be ordered
(Marradi 1990, 129).

2.      Typologies
When several fundamenta are jointly considered, a typology is produced. This may be done through
either intensional or extensional classification (Marradi 1990, 129).

3.      Taxonomies
When several fundamenta are considered in succession through a series of intensional
classifications, a taxonomy is produced. Specific concepts/terms (such as taxon, rank, clade) are
needed to deal with taxonomies (Marradi 1990, 129).

It is not difficult to find examples of the use of the terms classification, typology,
and taxonomy in disagreement with Marradi's definitions. Here, it will not be discussed whether or
not it is a good idea to use his definitions prescriptively.

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3.5 Typology

Typology is derived from the two Greek words typo meaning "type" and logos meaning "word." The
word typology literally means the study of types (subdivisions of particular kinds of things). We
saw above (3.4) that according to Marradi (1990) typologies are kinds of classifications in which
more than one fundamental criterion of division is simultaneously taken into account. Another
definition was provided by Kenneth D. Bailey:

Typology is another term for a classification. Two characteristics distinguish typologies from
generic classifications. A typology is generally multidimensional and conceptual. Typologies
generally are characterized by labels or names in their cells. (Bailey 1994, 4; italics in
original)

Bailey exemplifies:

As a hypothetical example, let us use two dimensions to construct a classification. These
dimensions are intelligence (dichotomized as intelligent/unintelligent) and motivation
(dichotomized as motivated/unmotivated). Combining these two dimensions creates a fourfold
typology; as shown in Table 1.1. These four categories can be defined as cells in the table. In
this case, they are types, or type concepts. A motivated and intelligent person can be labeled as
successful; an intelligent but unmotivated person is likely to be an underachiever; while a
motivated but unintelligent person is an overachiever; and one who lacks both intelligence and
motivation is likely doomed to failure. (Bailey 1994, 4; italics in original).

A hypothetical fourfold typology (after Bailey 1994, 5; Table 1.1):

                                             Motivated

                                            Unmotivated

                                            Intelligent

 Success, 1

 Underachiever, 2

                                           Unintelligent

 Overachiever, 3

 Failure, 4

The term typology is used in many fields. For example are Carl G. Jung's psychological types famous
(Jung 1971). In Library and Information Science (LIS) is typology used, for example about document
typologies. Web of Science, for example, distinguishes between article, book review, letter,
review, proceeding paper and other types of documents.

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3.6 Division

→ Logical division is sometimes seen as a concept closely related to classification, but is here
understood as one among other approaches to classification, more precisely, logical division is an
a-priori, rationalist approach to classification. Parry and Hacker (1991, 136-7):

Logical division and classification are closely related. In logical division, one starts with a
class (genus) and divide it into subclasses (but not into individuals). In classification, one
starts with individuals or classes, and groups them into classes on the basis of properties they
share. Like many abstract words ending in "-tion", "division" and "classification" are used both
for a process and for the product resulting from the process — in this case, a mental or verbal
activity, namely a hierarchical arrangement of concepts or terms. In the sense of such an
arrangement, every table that results from logical division may also be regarded as if it were the
result of classification.
As a process of knowledge, division is characteristically an a priori process based on meaning.
Classification, however, is primarily inductive, and may require empirical knowledge. In less
technical language, this means that a table of pure division can be constructed solely from the
meaning of terms in the table; no recourse to experience is necessary. I know, for example, that
squares are a subclass of parallelograms, not from experience but from the meanings of the terms
"square" and "parallelogram". On the other hand, if I engage in the process of classifying plants,
I would have to observe the similarities and dissimilarities of actual plants. I could not
construct a useful classification of plants unless I had observed the plants I wished to classify.
In the natural sciences, classification is far more important than division, since the principal
analysis and relationships are based on empirical data. Division plays an important tole in the
formal sciences of mathematics and logic, and in the empirical areas to which these disciplines can
be applied. But the formal scientist often uses methods partly inductive to develop the knowledge
that he will later expound deductively.

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4. Theories of classification [2]

In this section, the following theories are presented.
  * 4.1: The "classical view" of classes as defined by sets of elements with necessary and
sufficient attributes versus the views proposed by, derived from, or related to Ludwig
Wittgenstein's late philosophy. As opposed to necessary and sufficient attributes, these views
consider that classes and concepts are graded structures. This section briefly presents the
prototype theory suggested by Eleanor Rosch, as well as theories developed by Thomas Kuhn and
Michael Billig.
  * 4.2: The way of forming classes (e.g., by logical division, by measuring similarity among
elements, by collecting elements with a common ancestry, or by collocating tools to support human
activities) (the epistemology of classification). [3]
  * 4.3: The view that there is one correct or best classification versus the view that there are
different classifications for different purposes (the metaphysics of classification).

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4.1 The "classical view" versus "prototype theory" [4]

Aristotle developed a theory of classification in which all elements in a given class share at
least one characteristic with all other members. Classes should be designed so membership of a
class is given by a set of necessary and sufficient characteristics. For example, according to
Aristotle's per genus et differentiam definition, man is a rational animal. This definition first
considers a class or concept consisting of all animals (including humans). It then claims the
essential difference between humans (men) and all other animals is that humans are rational. In
this way, the class of animals is divided into two non-overlapping classes: rational
animals (humans) and non-rational animals (all other animals). In order for some organism to belong
to the class of humans, it is necessary that it is rational: all elements in the class must have
this characteristic. It is also a sufficient condition: if an organism has the attributes of all
other animals plus the attribute of being rational, it must be human (it is quite a different task
to find out if a given organism is rational or not). This has also been called monothetic
classification [5] or the feature theory of classification (for further information about the
classical view see, for example, Parry and Hacker 1991; Moss 1964; Frické 2016).

In the middle of the twentieth century, a new theory of classification was proposed, which
considered itself an alternative to the feature theory that had existed for millennia. Wittgenstein
(1953) claimed that not all concepts consist of elements which have a set of necessary or
sufficient characteristics. He used the metaphor of family resemblance for a series of overlapping
similarities, where no one feature is common to all the elements in the concept. In a family, some
members may be alike in one respect (e.g., the shape of the nose) while other members may be alike
in other respects. Wittgenstein's famous example is games, where he claimed that no characteristic
common to all kind of games exists. Experimental psychologist Eleanor Rosch (1978), inspired by
Ludwig Wittgenstein's later philosophy, first defined prototype theory [6]. Given the Roschian
theory, some elements are better representatives than others. For example, if the class or concept
is bird, in the classical theory this concept may be defined by attributes such as feathers, beak,
and the ability to fly, and every bird is as representative as any other. In the prototype theory,
on the other hand, a blackbird is considered a good example (at the least by Westerners), while a
penguin is considered a bad example. Instead of being defined by necessary or sufficient
characteristics, classes are determined by the overall likeness to a prototype — hence the name of
the theory.

A now well-established division between two theories of concepts and classification is therefore
classical or Aristotelian classification on the one side and prototype theory (or polythetic
classification) on the other. George Lakoff wrote about these two theories:

From the time of Aristotle to the later work of Wittgenstein, categories [and classification] were
thought to be well understood and unproblematic. They were assumed to be abstract containers, with
things either inside or outside the category. Things were assumed to be in the same category if and
only if they had certain properties in common. And the properties they had in common were taken as
defining the category.
This classical theory was not the result of empirical study. It was not even the subject of major
debate. It was a philosophical position arrived at on the basis of a priori speculation. Over the
centuries it simply became part of the background assumptions taken for granted in most scholarly
disciplines. In fact, until very recently, the classical theory of categories was not even thought
of as a theory. It was taught in most disciplines not as an empirical hypothesis but as an
unquestionable, definitional truth" [7] (Lakoff 1987, 6; emphasis in original).

Geoffrey C. Bowker provided another description:

An Aristotelian classification works according to a set of binary characteristics, which the object
being classified either presents or does not present. At each level of classification, enough
binary features are adduced to place any member of a given population into one, and only one class.
[...] Aristotelian models [...] have traditionally informed formal classification theory in a broad
range of sciences, including biological systematics, geology, and physics.
Rosch's (1978) prototype theory argues that, in daily life, our classifications tend to be much
fuzzier than we might at first think. We do not deal with a set of binary characteristics when we
decide that this thing we are sitting on is a chair. Indeed, it is possible to name a population of
objects that people would in general agree to call chairs that have no two binary features in
common" (Bowker 1998, 256).

There are still many people subscribing to the classical view, and Wittgenstein's view — and
thereby, indirectly, prototype theory — is criticized by Needham (1975), Sutcliffe (1993) and
Margolis (1994). See Fox (2011) for a recent discussion of prototype theory in knowledge
organization.

Thomas Kuhn [8] is well known for his book The structure of scientific revolutions (1962) in which
he made the concepts of "scientific paradigm" and "paradigm shift" widespread — including in common
language. It is less well known that his later research focused on concepts, classification, and
scientific taxonomies and inspired a new theory of concepts called "theory theory". The best
introduction to this work is probably that by Andersen et al. (2006), who wrote:

According to this theory, the basic conceptual structure of science is a classification system that
divides objects into groups according to similarity relations. The grouping is not determined by
identifying necessary and sufficient conditions, but by learning to identify similarities and
dissimilarities between the objects. It was one of Kuhn's central claims that one learns such
concepts by being guided through a series of encounters with objects that highlight the relations
of similarity and dissimilarity currently accepted by a particular community of concept users.
[...].
Kuhn's standard example of a learning process of this sort is a child learning the concepts "duck",
"goose", and "swan" (Kuhn 1974). In this example, an adult familiar with the classification of
waterfowl guides a child ("Johnny") through a series of ostensive acts until he learns to
distinguish ducks, geese, and swans. Johnny is shown various instances of all three concepts, being
told for each instance whether it is a duck, a goose, or a swan (Andersen et al. 2006, 20-21).

The most important aspects of Kuhn's theory are:
  * People learn concepts (or classifications) according to how these concepts are understood in a
given society by being confronted with exemplars and similarities as well as dissimilarities
compared with other concepts (e.g., by parents and teachers).
  * Two people can correctly identify the same concepts/classes even if they use different
characteristics to make the correct identification.
  * Dissimilarity plays as important a role as similarity in classification. Similarity alone is
not enough (see Andersen et al. 2006, 24ff).
  * A given concept/classification is based on a paradigm. For example:

  * Paradigm one: Ptolemaic astronomers might learn the concepts "star" and "planet" by having the
Sun, the Moon, and Mars pointed out as instances of the concept "planet" and some fixed stars as
instances of the concept "star".
  * Paradigm two: Copernicans might learn the concepts "star", "planet", and "satellites" by having
Mars and Jupiter pointed out as instances of the concept "planet", the Moon as an instance of the
concept "satellite", and the Sun and some fixed stars as instances of the concept "star". Thus, the
concepts "star", "planet", and "satellite" got a new meaning and astronomy got a new classification
of celestial bodies.

The difference before Copernicus, and later, say, Newton, is striking: after a paradigm shift we
learn to distinguish concepts in new ways (see Andersen et al. 1996).

A contemporary example: Ornithologists have recently discovered that the blackbird, which so far
has been considered one species, should be considered different species which have becoming similar
by adapting to the same ecological niche. In the future, we may therefore expect that not just
experts, but also amateur ornithologists, schoolchildren, and the rest of us learn to classify
birds in a new way.

Michael Billig, a social psychologist inspired by rhetoric, proposed another new theory that is
probably in harmony with Kuhn's view in important respects. Billig considers that thinking is like
a quiet internal argument. Therefore, psychological and rhetorical theories are closely linked, and
the psychology of classification/categorization can learn much from rhetoric. Billig (1996)
describes modern cognitive psychology's tendency to consider categorization a fundamental cognitive
process in both animals and humans. Modern cognitive psychology tends to view "[t]he individual as
an active processor of information" in which "the effect of a stimulus depends on how it is
categorized and interpreted by the perceiver" (quotations from Eiser 1980, 8). This basic
psychological process is often attributed a biological status by cognitive psychologists (this
criticism is also raised against Eleanor Rosch). Billig finds that the implication of this view is
that humans are tied to prejudiced and bureaucratic modes of thinking. Billig does not consider it
wrong that categorization is an important process, but from his studies in rhetoric he argues that
there must be two fundamental processes: categorization and particularization, the latter being a
reverse process in which something is not just considered an element of a class or a category, but
is considered something special. Billig further demonstrates with many examples how humans are able
not just to categorize and particularize but also to discuss and consider the way things are
categorized. Arguments about categories and particulars are important elements in human
communication and thinking, and they are often related to wide-ranging theoretical and ideological
issues and conflicts. By including particularization as a basic psychological process, Billig is
able to make room for people, who are not just prejudiced and bureaucratic but also open-minded and
flexible. In relation to research in knowledge organization, Billig's research raises serious
problems for the cognitive view that tries to base classifications on the study of the human mind.

The basic lessons from these new theories of classification may be summarized this way:
 1. Humans do not classify in a given way according to inborn or "given" characteristics, but
according to the human activities and goals that we have (which may require different
classifications).
 2. Instead of the classical model of sets of mutually exclusive and jointly exhaustive criteria,
we may need alternative models. Andersen et al. (2006) found that the so-called "dynamic frames"
represent the best way to represent classes. It has not been examined, however, whether this form
of knowledge representation is applicable or fruitful in relation to the construction of knowledge
organization systems (KOS) in LIS.

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4.2 The methodology of forming classes, the epistemology of classification

The fundamental elements of any classification are its theoretical commitments, basic units and the
criteria for ordering these basic units into a classification. (Hull 1998)

4.2a Elaine Svenonius (2004) proposed that three epistemological theories are important for
knowledge organization (or, as she preferred, knowledge representation):
  * α Operationalism
  * β The picture theory of meaning
  * γ The contextual or instrumental theory of meaning

These views may deserve their own entry in this encyclopedia. They are outlined below.

Concerning α, Svenonius raised the criticism that all operational definitions lack validity and
that operationalism represents a form of logical positivism.

Concerning β, what Svenonius termed "the referential or picture theory of meaning", she found that
this, too, "derives from an empiricist view of knowledge" (2004, 574). The author summarized the
basic problems with this theory:

First, the picture theory assumes a universal form of language in which the meaning of propositions
picturing the world are prescribed, relatively fixed, and generally understood. The objection here
is that pictures can be differently interpreted. A cup is half full or half empty. A picture of a
duck from another viewpoint could be a picture of a rabbit; a picture of a block could be
interpreted as a triangular prism.
Secondly, the picture theory implies fixity of reference. But the meanings of words are not
necessarily fixed in the sense of referring to sets of homogeneous objects in the real world or
clearly delineated mental concepts. Many words have fluid boundaries. (A chair with three legs is
still a chair.) Fluidity is necessary if words are to function in a variety of different contexts.
The picture theory falls down particularly in the case of abstract words whose referents are mental
constructs and function words, such as adverbial particles and prepositions.
A third problem with the picture theory is that it represents knowledge of the world as the
conjunction of knowledge of independent microworlds. To regard the totality of knowledge as a
simple aggregation is simplistic [...]" (Svenonius 2004, 578; note omitted).

Concerning γ, Svenonius's last theory was the contextual or instrumental theory of meaning. The
basic tenet of the instrumental theory of meaning is that we know what a word means when we know
how to use it. Svenonius found that this way of thinking led to adoption of the methods of
numerical taxonomy. However, numerical taxonomy may be considered a form of empiricism rather than
of the contextual or instrumental theory of meaning as developed by pragmatic philosophers. If this
understanding is true, we may conclude that Svenonius has not really suggested an alternative to
empiricism and logical positivism. Such alternatives are presented below (4.2c). Before this,
however, another important view will be presented.

4.2b Alberto Marradi distinguished the following senses of classification as an operation
(intellectual and otherwise):
  * α Intensional classification (or subdivision or downwards classification) [9]
"[T]he subdivision of the extension of a concept (genus) into several extensions corresponding to
as many concepts of lower generality (species). The former and all the latter concepts have the
same intension except for one aspect (fundamentum divisionis): on that account each species concept
is a different partial articulation of the genus concept" (Marradi 1990, 129; emphasis in
original).
  * β Extensional classification (or numerical taxonomy or upwards classification) [10]
"[T]he grouping of the objects/events of a set into several subsets according to the perceived
similarities of their states on one or more properties" (Marradi 1990, 129).
"[B]otanist Michel Adanson stated that "all parts and qualities, or properties and faculties of
plants ... barring not even one" ought to be considered before attempting a classification (1763,
clvi). Along with this idea, Adanson operated extensional [classification] and produced taxonomies
based on the rate of equal states on the total of properties considered between any two plants
(1763, vol. I)" (Marradi 1990, 136).
"The predominant criterion is to maximize homogeneity within classes and heterogeneity between
classes" (Marradi 1990, 135).
"Other labels have been proposed for the operation, over and above the old ones — "classification"
and "taxonomy". Among them "numerical taxonomy" (Sokal and Sneath 1963), "class formation"
(Capecchi and Möller 1968), "cluster analysis", etc." (Marradi 1990, 136).
  * γ Classing
"[T]he assignment of objects/events to classes defined by the first operation [subdivision] (or of
new objects/events to groups created by the second operation with other objects/events)" (Marradi
1990, 129).

Marradi's two first options are further discussed in the next section. His third option is not
related to a new fundamental method and is not further discussed in this article, but this issue is
partly dealt with in the entry on Subject. Forthcoming entries on subject analysis, indexing, etc.
will also cover this topic.

4.2c Birger Hjørland has suggested that there are four basic theories and approaches to
classification [11]: α rationalism; β empiricism; γ historicism, and δ pragmatism/critical theory.
All four will be presented and discussed below. The first two (rationalism and empiricism) are
related to theories already presented.
  * α Rationalism. Hjørland considers than the first of Marradi's operations, intensional
classification or subdivision, corresponds to what he in different writings has related to
rationalism:

Rationalist theories of indexing (such as Ranganathan's theory) suggest that subjects are
constructed logically from a fundamental set of categories. The basic method of subject analysis is
then "analytic-synthetic", to isolate a set of basic categories (=analysis) and then to construct
the subject of any given document by combining those categories according to some rules
(=synthesis). The application of rules such as logical division is by principle part of the
rationalist view (Hjørland 2011, 74).

According to Marradi, logical division dominated for centuries until challenged (or supplemented)
by extensional (empiricist) classification.

In our opinion, this belated development [of empiricist classification] depends on the fact that,
in order to be somehow formalized from a spontaneous activity into a respectable intellectual
operation within a scientific discipline, extensional cl. [classification] had to wait for the
development of another intellectual tool, viz. the idea of orderly recording the states of a vector
of objects on a vector of properties—in other words, for the intellectual forefather of what is
presently known as the data matrix (Marradi 1990, 135-136).

What are the major benefits and drawbacks of logical division as a method of classifying? Frické
(2016, 547) stated: "Logical division produces classifications with admirable qualities. Everything
has a place in a leaf, its own unique place, and the classification schedule embodies the maximum
amount of general information about the items being classified". Its weaknesses have been known for
a long time: "Aristotle had argued that logical division was an inappropriate tool for the
classification of organized beings" (Stevens 1998; electronic source, no pages). One limit of this
method is that it seems better suited to some kinds of object (e.g., formal objects, such as
mathematical objects) compared to other kinds of object (e.g., "organized beings"), but this is an
open issue today. A modern criticism is its relationship with essentialism. If the basis of
division (fundamentum divisionis: classification principle) is not to be arbitrary, it has to be
deduced from what are considered essential criteria, but this idea is heavily criticized today
(Wilkins 2013; Frické 2016).
  * β Empiricism. The second of Marradi's operations, extensional classification, has, according to
Marradi, also been termed "numerical taxonomy" and "cluster analysis", among others. It corresponds
to what Farradane (1950) termed "inductive classification" and Parrochia (2016) "phenomenal
classifications", and has been related by Hjørland to empiricism:

Empiricist theories of indexing are based on the idea that similar (informational) objects share a
large number of properties. Objects may be classified according to those properties, but this
should be based on neutral criteria, not on the selection of properties from theoretical points of
view because this introduces a kind of subjective criteria, which is not approved by empiricism.
Numerical statistical procedures are based on empiricist philosophy (Hjørland 2011, 74).

This means that overall likeliness, sometimes termed "phenetics" (e.g., statistical measures of
similarity based on a great number of attributes), is the basis of this method. This seems at first
to be properly scientific but, on further examination, it turns out to be based on a number of
problematic assumptions. The first question is about the number of properties needed.

A controversial question is how many properties are to be considered. Parsimony and elegance would
advise to keep that number down; on the other hand, it may be remarked that "increasing the number
of variables increases the probability of correct classification" (May 1982, 43). Since the concept
of "correct [classification]" is rather questionable [...], a better argument might be that, with a
formalized approach, there is no way to consider the information on the objects'/events' properties
unless those properties are included in the matrix. Therefore, leaving properties out of the matrix
entails a loss of information of unknown amount (Marradi 1990, 137).

If we assume that any object has an unlimited number of properties, then it is impossible to
consider them all, even in theory. By implication, any empirical classification is biased in ways
that cannot be controlled.

Another problem is that the descriptions of objects, on which this method is based, cannot be
atheoretical or objective (cf. Hjørland 2016a, 2016b) — or, as formulated by Gitelman (2013), "Raw
data is an oxymoron". In other words, the data used are always theoretically biased in ways we
often cannot recognize or control.

The third problem, closely related to the second, concerns the concept of similarity.
Classification has often been defined as bringing like things together (and thus separating unlike
things). "Likeness" is a concept that may also be expressed by other terms such as "similarity",
"sameness", "resemblance", or "equivalence". The problem is that things cannot be similar in an
objective way. Any object is similar to another object in some ways and dissimilar in other ways.
For any three objects, two different classifications can be constructed which fulfils the demand of
bringing the like objects together. Consider Figure 1 below, wherein the items may be classified
according to color or shape. None of those properties is objectively more important than the other.
For some purposes, the two squares are most alike and should be classified together. For other
purposes, the two black figures (a square and a triangle) are most alike and should be classified
together (see also Popper 1959, 441).

Figure 1: Classification criteria. Which two are similar?

By implication, empirical criteria for classification are not enough and we need some guidance on
how to determine which criteria should be used when determining similarity. If we take a biological
example, scientists have long recognized that modern-day birds and reptiles share a common
ancestor. Both groups lay shelled eggs and have scales (in birds, confined to the legs), nucleated
red blood cells, and a number of skeletal similarities. Different methods and paradigms in
biological taxonomy thus arrive at different results: Methods based on historical development
consider birds and reptiles as related groups (birds may be considered a kind of reptile), while
numerical taxonomy, based on a quantitative study of many structural similarities, considers birds
and reptiles to be less related (birds are not reptiles). The reason is that many animals develop
similar structures because they adapt to the same ecological niches, but may have very different
phylogenetic backgrounds and are by biologists considered different groups. (Considering the
reverse, in some spiders the male and the female are very different, and if numerical taxonomy is
used, the male and the female might be classified as different species, which obviously is
problematic.) Therefore, modern biological taxonomy is dominated by quite a different philosophy
and method: cladism. We shall return to this below. Consider, however, that Marradi saw these two
methods (logical division and phenetics) as exhaustive, but that Hjørland (2009) presented two
additional methods of classification related respectively to historicism and pragmatism/critical
theory. The biological examples should provide sufficient argument for the first of these, and
others have argued in a similar way. [12]
  * γ Historicist approaches to classification. To say that two elements belong to the same class
(or "clade") if they share a common ancestor is clearly different from defining membership of a
class by similarity (sets of characteristics as arranged by logical division or numerical
taxonomy). Today, this is the dominant approach in biological systematics. This approach (termed
"cladistics", "phylogenetic classification", "historical classification", → "genealogical
classification", or "genetic classification") is based on the historical or evolutionary
development of the classified objects (Hennig 1966; Hjørland 2003, 107; Gnoli 2006). It is not only
used in biology, but also, for example, for classification of languages and musical instruments —
and should be considered one of four general approaches to classification. Hjørland (2013, 2016a)
suggested genealogical classification may also be used in bibliometrics and information retrieval
as an alternative to classifying documents according to similarity. To define membership of
classes, clades, or species by common ancestor is different from, but related to, an evolutionary
ordering of classes (see Dousa 2009 for early discussion of evolutionary order in library
classification).

Although cladism seems to dominate biological taxonomy today, it has also met with skepticism:

[I]t is at the same time becoming clearer that there is very likely no such ideal classification.
There is no reason why a classification that reflects the origins of the things classified should
coincide exactly with one aimed at the ecological relations of those things, and it is increasingly
perceived that these can and do diverge (Dupré 2002, chs. 3-4). This possibility becomes even
clearer in view of the difficulties that are emerging in the project of evolutionary-based
classification. Speciation was once seen as an all or nothing affair leading to complete isolation
of one group from another. It is now clear that for micro-organisms, in particular, there is very
little such isolation, and genetic material moves in many ways from one kind of organism to
another. In fact it has become common to conceive of the genome of an ecosystem (the soil of an
area, or a body of water) rather than the privatized genome of an individual organism (e.g. Venter
et al. 2004). The classical picture of speciation applies quite well to some of the most complex
multi-celled organisms, such as mammals and birds, though much less well to plants. An important
movement in biology is to transcend the anthropocentrism that takes the peculiarities of our own
corner of the living world as the model for all (Dupré 2006, 31).

Dupré's reservations are not about the validity of the cladistics approach as such, but about the
idea of one ideal classification. It seems clear that genetic classification is a distinct
approach, with some major benefits — to know about the origin of things is to know things in a
deeper way than just to know about sets of attributes (which, in the historical perspective, often
looks superficial).

There is also a subjective side of classification, and this subjectivity has developed
historically. The classifying subject is influenced by his or her culture, paradigms, and
tradition. This may be termed, for example, hermeneutics, historicism, or social epistemology. This
can be fruitfully illustrated in the classification of colors:

[W]hen the colour vocabularies of various languages are considered and compared, the researcher
finds that there are many different ways in which humans categorize and "label" colours, resulting
in an amazing array of misunderstandings. Monoglot individuals invariably believe that their own
colour system is clear and obvious, and they are often mystified when confronted with an
alternative system. So the first step which the reader has to take when entering the world of
colour semantics is probably the most difficult of all; s/he must restrict his or her own colour
system to normal, everyday speech, and learn to set it aside when considering foreign or historical
colour descriptions. The aim is to dispose of any preconceptions about how colour "should" be
classified and described, so as to gain insights into the workings of other languages and cultures,
and into the nature of colour itself (Biggam 2015, 1).

Formerly, Hjørland wrote:

[H]ermeneutical theories of indexing suggest that the subject of a given document is relative to a
given discourse or domain and is why the indexing should reflect the need of a particular discourse
or domain. According to hermeneutics, a document is always written and interpreted from a
particular horizon [note omitted]. The same is the case with systems of knowledge organization and
with all users searching such systems. Any question put to such a system is put from a particular
horizon. All those horizons may be more or less in consensus or in conflict. To index a document is
to try to contribute to the retrieval of "relevant" documents by knowing about those different
horizons (Hjørland 2011, 74).

Historicism is therefore, as we have seen, an approach that may be applied to both the object and
the subject in classification. If both the object and the subject are considered, we may speak of a
united historicist theory. B. M. Kedrow presents such a united historicist view of classification:

Historicism as a key to any natural classification.
Of crucial importance for the analysis of the problem at hand is the historical approach to its
consideration and solution, in other words, the principle of historicism. This refers both to the
development history of the objects studied by the sciences as well as to the evolution of
scientific knowledge itself.
In fact, any artificial classification of things or knowledge of things is mainly characterized by
the lack of historicism. In that case, the relationships between things and between things and the
knowledge of them will not reveal the classification as a result necessarily incurred in the course
of development but rather as random and superficial relations that sometimes are also fixed by man
himself in order to understand the given material.
In contrast, the truly natural classification reflects the real connections between objects as they
have evolved in their development or in the development of knowledge of them. This was, for
example, the case with the classification of the chemical elements. This truly natural system could
only be made after the relationships between the elements had been established as real
interactions, which had found static relations and given tabular form. All previously established
systems of elements proved to constantly be one-sided, in essence artificially, in a sense,
arbitrary, because none of them were based on the principle of historicism (Kedrow 1975, vol. 1,
4-5; translated from German by BH. Concerning natural versus artificial classification, see
section 4.3a).

A more recent view was expressed by Fulvio Mazzocchi:

Hermeneutics and postpositivist epistemology emphasized, respectively, the historicity of
understanding and the incommensurability [13] of alternative scientific paradigms. Postmodernist
theories argued for the breakdown of "grand narratives", indicating the need to embrace pluralistic
views. What is basically common to many of these approaches is the refusal of the belief that an
absolute vantage point can be reached. There is no ultimate criterion for univocally distinguishing
accidental from distinctive features: the fixing of such a distinctiveness always depends on a
given perspective (Mazzocchi 2017, 373).

The most concrete implication of the united historicist view of classification is that theories
become important in the explanation for our categories compared to similarity or other criteria
(cf., Murphy and Medin 1985; Hjørland and Nissen Pedersen 2005). Different theories or paradigms
imply different classifications; therefore to provide design principles for classifications is to
negotiate the different theoretical influences on the domain to be classified. Whereas empiricists
and positivists tends to "let the data speak for themselves", the hermeneutics-oriented researcher
tends to apply a broad orientation which is able to uncover the theoretical influences that have
produced the data and their interpretations and classifications in a given domain. This issue
brings us to the last of the basic approaches to classification: pragmatic and critical theories.
  * δ Pragmatic and critical approaches to classification. The pragmatic and critical approach to
classification is based on considering the goals, values, interests, policies, and consequences of
classification. There may be many different values at play in forming classifications [14].
Classifications based on this approach are constructed in order to support explicit interests. From
this perspective, a classification can never be neutral, but will always tend to support certain
goals and interests at the expense of other interests. Nobes and Stadler (2013) examined "how the
classifiers themselves and the characteristics that they choose can affect classification" [15] and
they observed "that objectivity is neither possible nor desirable in classification. Despite the
arbitrariness, some classifications can be more reasonable or more useful than others" (573).
Different interests and kinds of subjectivity may not be explicit (or they may be in conflict with
their stated goals) and it is therefore an important task to uncover the hidden assumptions in
classification principles, classification criteria, and in all kinds of knowledge organization
systems (KOS) and information retrieval (IR). As formerly stated:

Pragmatic and critical theories of indexing are in agreement with the historicist point of view
that subjects are relative to specific discourses but emphasize that subject analysis should
support given goals and values and should consider the consequences of indexing. These theories
emphasize that indexing cannot be neutral and that it is a wrong goal to try to index in a neutral
way. Indexing is an act (and computer-based indexing is acting according to the programmer's
intentions). Acts serve human goals. Libraries and information services [and classifications] also
serve human goals, and this is why their indexing should be done in a way that supports these
(Hjørland 2011, 74).

Jesse Shera expressed the pragmatic approach very clearly:

The pragmatic approach to classification through meaningful units of knowledge must be based on
recognition of the obvious truth that any single unit may be meaningful in any number of different
relationships depending on the immediate purpose. Thus it is the external relations, the
environment, of the concept that are all-important in the act of classifying. A tree is an organism
to the botanist, an esthetic entity to the landscape architect, a manifestation of Divine
benevolence to the theologian, a source of potential income to the lumberman. Pragmatic
classification, then, denies the existence of the "essence" of tree, for each of these
relationships owes its existence to different properties of the tree. Relationship is not a
universal, but a specific fact unique to the things related, and just as these relations reveal the
nature of the relata, so the relata determine the character of the relationship. (Shera 1951,
83-84; italics in original).

In recent years, there has been a focus on ethical issues in knowledge organization (e.g., Adler
and Tennis 2013), as well as on the consequences of classification (e.g., Bowker and Star 1999) and
feminist approaches to knowledge organization (e.g., Fox and Olson 2012). Together with research,
uncovering hidden assumptions in classification and arguing about the paradox of atheoretical
classification (e.g., Hjørland 2016b), the pragmatic/critical approach is exemplified.

One may wonder, however, if pragmatic/critical classification is scientific or able to function as
the theoretical basis for classification research and practice. The first impression might be that
this is a decline in scientific progress. For example, to classify animals in relation to human
interests as domestic animals, pets, and pests seems primitive compared with biological
classification in which no such interests and goals seem to be used. The answer to this argument is
that there are levels of pragmatic classification and that basic science can be interpreted from
the perspective of pragmatic philosophy. Pragmatism may be more or less short term or long term,
and the pragmatic value of cladism should be understood from the perspective of long-term
interests. Critical theory claims that, in the end, our scientific theories should be evaluated
from the perspective of human practice. One of the reasons for considering pragmatism/critical
theory as a serious approach is that the other approaches (rationalism, empiricism, and
historicism) have problems that are not less significant. We cannot have a science of
classification without considering criteria for classification as the fundamental problem. The
claim of pragmatism and critical theory is that, in the end, such criteria must serve human values
and interests (see Pihlström 2009 for scholarly argumentation). It is extremely important to
realize, however, that truth is always the goal in science and scholarship. Worst of all is the
manipulation of research in order to serve some specific interests. Like the historicist approach,
the pragmatic/critical approach bases design principles for classification on the negotiation of
the different theoretical influences on the domain to be classified, but it provides some
additional criteria for theory analysis and evaluation, such as the social conditions under which
knowledge is being produced.

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4.3 Some metaphysical issues of classification: is there one correct classification?

Has the world one unique structure ("taxonomic monism"), or is there more than one structural
entity and process ("taxonomic pluralism")? Are the structures of the world mind-independent
(realism), or are they artefacts projected into the world (idealism)? Can our classifications be
natural, or are they always artificial? These are core issues in the metaphysics of classification.
As stated by Anjan Chakravartty:

The preeminent question of the metaphysics of classification is that of whether the world is itself
naturally subdivided into kinds of things. Are kinds out there, so to speak, or are they rather
artefacts of convention, existing only insofar as classificatory practices are brought to bear by
creatures such as ourselves? (Chakravartty 2011, 157)

We shall here present the following issues: 4.3a Artificial versus natural classification, 4.3b
Order versus disorder of things and unity versus disunity of knowledge

4.3a Artificial versus natural classification. Natural classification can be expressed by Plato's
metaphor of "carving nature at its joints" (Plato c.370 BC, Phaedrus 265e; see also Campbell et
al. 2012). Carl Linnaeus is, however, often recognized as the first scholar to clearly have
differentiated "artificial" and "natural" classifications:

As far as I can see, Linnaeus was the first to draw a clear terminological distinction between
artificial and natural systems, and this was praised as one of his main achievements by later
naturalists and philosophers" (Müller-Wille 2007, 550; cf. Müller-Wille 2013, 311).

Although Linnaeus considered natural classification the ideal, he recognized that his own system
(at least partly) represented an artificial classification.

Linnaeus realized that natural orders could not be defined. Even the most "natural", such as the
Umbelliferae, the carrot family, lacked features that were unique to and constant within them.
Until these were found, natural groups were "like a bell without a clapper"; in modern parlance,
they were polythetic (Stevens 1998).

However, the meaning of ‘natural classification' has often been considered unclear:

In the middle of the seventeenth century, many, like Aristotle 2000 years before, believed in a
nature that could be represented as some version of the scala naturae, a linear sequence of
organisms arranged according to ideas of "highness" and "lowness", in which man was above all
organisms (and often not part of nature), and angels and ultimately god [sic] might be above him.
There were many other ways of representing nature, and as the geologist Francis Bather observed in
1927, "not a single naturalist had a clear idea of what he meant by 'natural'. All he knew was that
the other fellow's classification was unnatural" (Bather 1927). In early usage, natural history
itself for the most part had no historical element; "history" meant "story" or "description"
(Stevens 2016, 494).

John Stuart Mill suggested the following definition:

The Linnæan arrangement answers the purpose of making us think together of all those kinds of
plants, which possess the same number of stamens and pistils; but to think of them in that manner
is of little use, since we seldom have anything to affirm in common of the plants which have a
given number of stamens and pistils (Mill 1872, 498).

The ends of scientific classification are best answered, when the objects are formed into groups
respecting which a greater number of general propositions can be made, and those propositions more
important, than could be made respecting any other groups into which the same things could be
distributed [...]
A classification thus formed is properly scientific or philosophical, and is commonly called a
Natural, in contradistinction to a Technical or Artificial, classification or arrangement (Mill
1872, 499).

Alphabetical arrangement is also a kind of artificial classification because it is not based on
relationships between objects, but on formal characteristics of the names of objects. Artificial
classifications often serve important but limited practical purposes, whereas natural
classifications have broader application functions. (A yet more limited kind of artificial
classification is "ad-hoc classification", which is just made for a specific task,
non-generalizable, and not intended to be able to be adapted to other purposes: cf. Hudon et al.
2005). Marradi found, however, that use of the term "natural classification" is often connected
with a problematic positivist view. [16]

William Parry and Edward Hacker explained the concept in accordance with John Stuart Mill's
understanding:

For example, one may divide rocks — or even animals — into those weighing less than ten grams,
those weighing at least ten but less than twenty grams, and so on; but this is likely to be of
little use, except perhaps for knowing what it would cost to mail them (Parry and Hacker 1991,
133).
And later:
[A classification] is fruitful to the extent that it suggests new hypotheses, explanations, and
theories concerning its subject matter. For example, the periodic table — the classification of the
elements — proved extremely fruitful, since it suggested the existence of hitherto unknown elements
and even suggested what physical properties they would have. It should be noted that natural
classifications, by definition, are more fruitful than artificial ones (Parry and Hacker 1991,
139).

Hjørland (2016b) considered the classification of mental disorders in the DSM system. The third
edition of this system especially claimed to be atheoretical and tended to give priority to
reliability in diagnosis rather than in the validity of classifications. This creates a system with
doubtful functions in the understanding and treatment of mental diseases. While it is relatively
easy to make a classification reliable (e.g., by classing according to weight, as in Parry and
Hacker's quote), it is much more difficult, but also much more important, to make a classification
useful for predicting the outcome of interventions. Therefore, the distinction between artificial
and natural classification is important when natural is understood as a classification based on a
substantial theory — implying that competing theories provide competing natural classifications.

There are two additional aspects to consider in relation to natural classification: the concepts of
"natural kind" and "naturalism in classification".

A natural kind can be understood as a grouping that reflects the structure of the natural world
rather than the interests and actions of human beings. Chemical elements — e.g., gold — are often
taken as an example of a natural kind. However, the philosophical problems of natural kinds, and
how it can be decided if something is or is not a natural kind, are big ones (obviously, the social
constructivist denies that natural kinds exist). Here we shall not go further into this issue but
leave it to a hopefully forthcoming independent article. There is a large literature on this
concept: see, for example, Khalidi (2013).

Naturalism in classification may be understood as a general approach to classification theory that
establishes a close connection between knowledge organization and classification in empirical
science and scholarship (e.g., biological classification, classification of the chemical and
physical elements, classification in arts, linguistics, psychiatry, etc.). Naturalism is therefore
opposed to the idea that the field of knowledge organization has a set of a priori classification
principles or methods. Naturalism in classification is based on the corresponding concept of
"naturalistic epistemology", which has been described the following way:

Broadly speaking, however, proponents of NE [naturalistic epistemology] take the attitude that
there should be a close connection between philosophical investigation — here, of such things as
knowledge, justification, rationality, etc. — and empirical ("natural") science (Rysiew 2016).

Naturalistic classification is therefore the attempt to learn classificatory principles by studying
how the most successful classifications have been constructed in different domains, as well as the
discourses and controversies about classification and its philosophy. In the field of knowledge
organization, this has been relatively neglected because the field has tended to provide
prescriptive principles on how to classify knowledge (e.g., facet analytic principles, standards
for thesaurus construction, or user-based methodologies).

4.3b Order versus disorder of things and unity versus disunity of knowledge. It has been (and
probably still is) characteristic of many researchers to believe in a fundamental order underlying
the apparently confusing empirical picture of the world. Related to this view is a belief in the
nature of knowledge to reflect or converge toward this underlying order. The first point is about
taxonomic monism versus pluralism; the second is about descriptive monism versus pluralism (or
epistemic relativism). Often taxonomic monism is associated with scientific realism, while
taxonomic pluralism is associated with relativism. Chakravartty (2011) argued, however, that
taxonomic monism is in opposition to contemporary science and that a form of taxonomic pluralism is
consistent with realism.

Henry Bliss is a library science representative holding the belief in an underlying order of things
and in the unity of and consensus of knowledge. He wrote:

The more definite the concepts, the relations, and the principles of science, philosophy, and
education become, the clearer and more stable the order of the sciences and studies in relation to
learning and to life; and so the scientific and educational consensus becomes more dominant and
more permanent (Bliss 1933, 37).

A critique of this view was made by Satija (1992, 40-41), paraphrasing McGarry (1991, 148):

Knowledge is a cultural entity and keeps shifting its pattern like a kaleidoscope. An emergence of
the new knowledge modifies the structure of the whole. Contrary to H. E. Bliss (1870-1955) there is
no permanent order in knowledge. "Pattern is new every moment", said T.S. Eliot (1888-1965), with a
poetic vision.

In information science and knowledge organization, this tension between the idea of order and
stability and the reality of disorder and relativity is clear in the differences between
traditional bibliographical classifications on the one hand, and the bibliometric maps based on,
for example, co-citation patterns on the other. Whereas traditional classifications tend to provide
relatively stable structures, the citation practices of researchers tend to display very unstable
patterns.

While the goal of knowledge organization is to discover or construe some kind of order, the nature
of the order matters. Francis Miksa, for example, wrote: "In the end, there is strong indication
that Ranganathan's use of faceted structure of subjects may well have represented his need to find
more order and regularity, in the realm of subjects, than actually exist" (Miksa 1998, 73). This
quote may be considered a criticism of Ranganathan in accordance with Hjørland's (2014) criticism
of rationalism.

Jens-Erik Mai discussed this from the perspective of post- or late modernist philosophy:

This paper traces and interrogates the shift from classification-as-ontology, in which everything
is defined as it is, to a more contemporary notion of classification-as-epistemology, in which
everything is interpreted as it could be — or more precisely, the paper argues for a conceptual
move from modern monistic ontology to late-modern pluralistic epistemological foundation for
classification theory and practice (Mai 2011, 711).

This opens many questions, and the most important claims by Mai may be considered the critique of
the positivist view that the researcher and knowledge organizer are neutral agents providing an
objective mirror of the universe of knowledge (see also Pando and de Almeida 2016).

The same issue was also addressed by philosopher Finn Collin, who discussed a social constructivist
view of classification, which he formulated in this way:

to isolate a certain kind of thing is the same process as classifying individual things. And
classification is a matter of sorting things into groups, the members of which are more similar to
each other than to items outside the group. However, things are only similar or dissimilar in
certain respects [...]. Classifications are not objective divisions, inherent in the nature of
things, but are structures we impose upon nature. [...] kinds of things are indeed human creations
(Collin 1993, 29: italics in original).

Then Collin wrote (29): "I believe this reasoning is mistaken. What follows from the premises is a
less radical conclusion." Later, he summed up his argument:

the nominalist argument mistakes a valid anti-essentialist point for an anti-realist one. It is
true that there is not, among the true descriptions of a thing, one which is privileged, in the
sense that any classification of the thing has to be based upon that particular description. There
is no uniquely correct classification of a thing, one that shows what the thing really is,
rendering alternative classifications somehow misleading or inappropriate. But it is a mistake to
infer from this that things do not in themselves belong to any classifications at all and that
things only come to belong to classes when we place them there. Once we relativise similarity and
dissimilarity to particular aspects of things, similarity and dissimilarity turn out to be
objective, although relational, properties of things, and the predicates that are defined by the
equivalency classes of things turn out to be genuine properties of those things. Reality
possesses all the properties attributed to it in these alternative descriptions" (Collin 1993, 43;
italics in original).

An issue regarding Collin's argumentation is "the true descriptions of a thing". Because
descriptions are theory-dependent, different descriptions are not necessarily equally true. Also,
of course, classifications are made, chosen, or used for a purpose, and therefore our
classifications to a large degree will be human creations (but not therefore arbitrary or
contingent; within ornithology, for example, there seems now to be considerable optimism that a
"final" classification of birds on the overall level seems within reach; cf. Fjeldså 2013). [17]

Mazzocchi wrote about the opposite of one right way to "carve nature at its joints", taxonomic
pluralism:

Ontological pluralist views have also been developed. For instance, Dupré's (1993) "promiscuous
realism" conceives the world (his argument refers, above all, to the biological realm) as made up
of a multidimensional complexity: things are interconnected and interrelated to one another in
multiple ways; there is no unique way of carving nature at its joints or one ultimately right way
of classifying or hierarchizing. Rather, there are many equally legitimate ways of dividing the
world into "kinds", depending on the purposes of investigations.
Returning to Greek philosophy for a moment, we find that even Aristotle, with respect to zoological
classification, supported a pluralist view (see Parts of animals I and History of animals) that
partially resemblances Dupré's view (Henry 2011). The biological world contains natural kinds
marked by real, objective boundaries, but at the same time it is not possible to assign animals to
a unique set of mutually exclusive and non-overlapping kinds. Even in Aristotle's view, many
cross-cutting joints can be found in nature. Which joints are chosen to be cut along depends at
least partially on the explanatory context. Since there are diverse explanatory purposes in
zoology, organisms can be grouped into various (cross-cutting) kinds (Mazzocchi 2017; see
also Galison and Stump 1996).

In this connection, it is worth mentioning the mental models or metaphors that govern our view of
how knowledge is organized.

The tree model has been historically associated with the philosophical position of classical
realism. A number of basic assumptions underlying such a position can be listed. Ontologically,
there is: (1) a reality that exists independently of us, (2) a single set of categories, and (3) a
single set of essential properties and therefore a unique way of dividing the world into kinds.
Epistemologically, saying that also implicitly means that: (4) the ultimate order of the world is
within the reach of human cognitive means, i.e. there is an absolute or neutral vantage point from
which to grasp reality as it "actually" is, and this vantage point is accessible to us.
Such a model has had a strong influence on logic, philosophy, and science. In Western culture, the
tree structure based on Aristotle's logic has been the dominant model of classification. Scientific
taxonomic thinking (for instance, the Linnaean classification system in biology) and many
contemporary semantics theories (for instance, Chomsky's sentence diagrams) also embody a similar
scheme" (Mazzocchi 2017, 372).

The tree metaphor is increasingly ousted by alternatives such as net and the rhizome (see Mazzocchi
2013) and it matters which models guide our research and practice.

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Conclusion

The concept of classification and its associated theories is extremely wide-ranging and
interdisciplinary. Many kinds of knowledge must be combined in order to make overall progress in
this field. Such projects may be guided by different basic assumptions. Daniel Parrochia seems to
base his suggestion for such a program on more formal and mathematical approaches. In 2016 he
wrote:

In spite of these advances, most of classifications are still based on the evaluation of
resemblances between objects that constitute the empirical data. This one is almost always computed
by the means of some notion of distance and of some algorithms of aggregation of classes. So all
these classifications remain, for technical and epistemological reasons that are detailed below,
very unstable ones. A real algebra of classifications, which could explain their properties and the
relations existing between them, is lacking. Though the aim of a general theory of classifications
is surely a wishful thought, some recent conjecture gives the hope that the existence of a
metaclassification (or classification of all classification schemes) is possible (Parrochia 2016).

Alternatively, another program may be suggested (not necessarily in conflict with Parrochia's).
This program is less formal (and therefore more substantial) and views classifications as tied to
(domain) theories. By implication, the study of classification involves the study of theories in
different domains and the ontological claims of those theories. The justification of a good
classification in this perspective is to make a justification of the theoretical premises on which
it is based.

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Acknowledgements

I am very grateful to Fulvio Mazzocchi, who served as the editor of this article. He and the two
anonymous referees provided detailed, knowledgeable, careful, and fruitful suggestions for
improving the original manuscript. Also thanks to Daniel Parrochia for positive feedback on the
manuscript.

Endnotes

1. Wesolek (2012, 1) stated: "He [Hjørland] thinks that concept classification should not strive to
classify on the basis of the properties of objects, but rather on descriptions of objects that are
loosely derived from human activity and social negotiation." In order to classify by properties, we
have to know these properties and the argument is that we only know them from descriptions — our
own or those of other people (or from perceptions, but such perceptions have to be transferred to
descriptions) — and those descriptions or perceptions will be influenced by culture, goals,
interests — in short, subjectivity. In this way, classification is always based on properties of
objects. Hjørland's definition of classification is, however, correctly cited by Wesolek on the
same page: "Classification, as defined by Hjørland, is the 'sorting of objects based on some
criteria selected among the properties of the classified objects'."

2. Parrochia and Neuville's (2013) monograph Towards a general theory of classifications is written
from the perspective of mathematics but demonstrates surprisingly broad knowledge of classification
research, including research in the community of knowledge organization (see also Parrochia 2016).
They, too, find that their work is about the epistemology of classifications, not a mathematical
textbook or monograph (vii). They write: "[T]he least we can say is that the field of a
mathematical theory of classifications is not a completely stable domain, and one is led to think,
finally, that a vast side of it is still to be developed. Another reason for the lack of a general
theory, close to the previous one, is that scientists are faced with a very difficult problem
(finding a formalism enough general to apply to any kind of classifications), for which no complete
solution is known at the present. All the same, we think that the research we have carried on for
more than thirty years might be of some interest for librarians, logicians, and also for scientists
in the different fields of empirical science, all of whom need to devise their own classifications.
But this book has a deeper stake. In fact, pure mathematics wants also a general theory of
classifications to take over from classic (and too limited) versions of set theory..." (Parrochia
and Neuville 2013, xv). Parrochia and Neuville (2013) seem to assume that a general theory of
classification(s) must be a mathematical theory, but do not discuss if other views may also be
important, or what the relative contribution of mathematics is. No doubt mathematics is important,
but it is certainly not all there is to say about classification.

3. An anonymous reviewer wrote: "It is a bit funny to call the methods of forming classes
‘epistemology' (which is theory of knowledge). If anything, it would be 'ontology'." However, the
four methodologies suggested in this article are rationalism, empiricism, historicism, and
pragmatism, which are well known epistemological positions, and thus normative principles on how to
obtain knowledge. It is correct, however, that these positions also rest on different ontological
assumptions. The a priori of rationalism is clear ideas or logical units; for empiricism, it is
sense impressions; for historicism, it is change; and, for pragmatism, the a priori is living and
acting in the world.

4. An anonymous reviewer wrote: "The explanations of the Classical Theory and Prototype Theory are
not strong" and provided a helpful, but lengthy improved description. The choice made here is to
try to maintain the short outlines here and later to have these theories covered by independent
articles.

5. A monothetic class is defined in terms of characteristics that are both necessary and sufficient
in order to identify members of that class. This way of defining a class is also termed the
Aristotelian definition of a class. A polythetic class is defined in terms of a broad set of
criteria that are neither necessary nor sufficient. Each member of the category must possess a
certain minimal number of defining characteristics, but none of the features has to be found in
each member of the category. This way of defining classes is associated with Wittgenstein's concept
of "family resemblances". The monothetic type is a type in which all members are identical in all
characteristics; the polythetic type is a type in which all members are similar, but not identical.
The distinction between monothetic and polythetic classification is discussed by Rijsbergen (1979,
28-29): "An early statement of the distinction between monothetic and polythetic is given by
Beckner (1959, 22): 'A class is ordinarily defined by reference to a set of properties which are
both necessary and sufficient (by stipulation) for membership in the class. It is possible,
however, to define a group K in terms of a set G of properties f1, f2, . . . , fn in a different
manner. Suppose we have an aggregate of individuals (we shall not yet call them a class) such that
(1) each one possesses a large (but unspecified) number of the properties in G;
(2) each f in G is possessed by large number of these individuals; and
(3) no f in G is possessed by every individual in the aggregate.'
The first sentence of Beckner's statement refers to the classical Aristotelian definition of a
class, which is now termed monothetic. The second part defines polythetic. [...]
To illustrate the basic distinction, consider the following example (Figure 3.1) of 8 individuals
(1-8) and 8 properties (A-H). The possession of a property is indicated by a plus sign. The
individuals 1-4 constitute a polythetic group, each individual possessing three out of four of the
properties A,B,C,D. The other 4 individuals can be split into two monothetic classes, {5,6} and
{7,8}. The distinction between monothetic and polythetic is a particularly easy one to make
providing the properties are of a simple kind, e.g., binary state attributes. When the properties
are more complex, the definitions are rather more difficult to apply, and in any case are rather
arbitrary.

Bowker (1998, 256) wrote: "Aristotelian models — monothetic or polythetic — have traditionally
informed formal classification theory in a broad range of sciences, including biological
systematics, geology, and physics". The first part of this quote was eliminated from the same quote
in section 4.1 because it seems confusing and probably wrong that polythetic classification is part
of formal classification theory and has been termed Aristotelian.

6. The idea is older, however. "The starting point is the work of the great French botanist Michel
Adanson, who proposed that a member of a class of plants did not need to possess all the defining
features of the class, and that a deviant specimen did not need to be assigned to a separate class
(Adanson 1763, i: cliv sqq.) [note omitted]. 'The important point he made was that creatures should
be grouped together on the greatest number of features in common, and there is no justification for
deciding a priori on the relative importance of characters in making a natural taxonomy' (Sneath
1962: 292)" (Needham 1975, 353).

7. An anonymous reviewer wrote: "Just as a mild correction to Lakoff's verbal flourishes, the
Aristotelian-Classical theory certainly had been the subject of major debate, a debate that had
lasted 2000 years and involved some of the finest scholars."

8. An anonymous reviewer wrote: "I am not sure about Kuhn and Billig in this setting. We are
getting drawn off into potentially quite deep cognitive or social psychology. But isn't our
interest storing and retrieving information or knowledge?" Yes, that is our interest, and the claim
is that Kuhn and Billig provide important knowledge for this purpose. Kuhn provides the knowledge
that we need to consider how different theories or paradigms classify knowledge and relate our
decisions to a choice or a negotiation between different views. Billig help us see the problematic
assumptions in the cognitive view according to which we have some built-in mechanisms on how to
classify knowledge.

9. "From Cesalpino to Linnaeus, this [downward classification by logical division] was the almost
universally preferred system, particularly in botany (Mayr 1982, 158-179). In this methodology, the
classifier starts with the entire 'universe' — let's say, all animals — and, with the help of
divisional logic, divides them into more and more homogeneous groups. A criterion like blood
temperature, when applied to animals, results in two groups: warm-blooded and cold-blooded animals.
By continuing dichotomy, one finally reaches the species level. Even though Linnaeus in his
artificial system still employed downward classification, it had become evident by his time that a
reliance on single characters — and the inevitable arbitrariness of the sequence in which these
characters were chosen — could lead to rather artificial systems. At the end of the eighteenth
century, downward classification was therefore replaced by upward classification. In retrospect, it
eventually became evident that the downward procedure actually produces identification schemes
rather than classifications. It survives today in the form of keys. Curiously, in the literature of
the philosophy of science, particularly of logic, classification by logical division has been
considered the method of classification up to modern times" (Mayr 1995, 420-421).

10. "This system [grouping or upwards classification] most closely conveys the basic meaning of the
term classification, that is, to assemble items into classes on the basis of resemblance in
observed characters" (Mayr 1995, 421) and "After earlier trials by some herbalists and by Magnol,
it was particularly Adanson (1763) who promoted classification by grouping. By the first third of
the nineteenth century it had become the almost universal method of classifying plants and animals
(Mayr 1982, 190-208). The empirical rule guiding the taxonomist was well stated by Whewell (1840,
vol. 1, 521): 'The maxim by which all systems professing to be natural must be tested is this: that
the arrangement obtained from one set of characters coincides with the arrangement obtained from
another set'" (Mayr 1995, 422).

11. An anonymous reviewer wrote: "There wants to be emphasis here that the classification is
systematic classification [as opposed to conceptual classification]." However, the suggested
principles are meant to serve conceptual classification as well as systematic classification, and a
fundamental view is that conceptual classification represents the core theory on which systematic
classification is based. The reviewer seems to view the two kinds of classification as too
dualistic.

12. Medin and Aguilar (1999, 104; emphasis in original), for example, wrote: "Why is this notion
that categories are defined by some "objective" similarity controversial? The main criticism has
been that the notion of similarity is too unconstrained to be useful as an explanatory principle
(Goodman 1972; Murphy and Medin 1985). Similarity is usually defined in terms of shared properties,
but Goodman argued that any two things share an unlimited number of properties (e.g. robins and
elephants can move, weight more than two ounces, take up space, can be thought about, etc.). Given
this apparent flexibility, it may be that we see things as similar because they belong to the same
category and not vice versa. That is, maybe we can explain similarity in terms of categories."

13. About incommensurability, see Kuhn (2000), and consider the following quotes.
"Incommensurability arises because it is impossible to transfer the natural categories employed
within one taxonomic structure into the categorical system of another such structure. Apparently on
the basis of such taxonomic incommensurability, Kuhn asserted a number of antirealist theses about
truth, reference and reality. In this paper, it will be argued, however, that, far from leading to
antirealist consequences about the relationship between theory and reality, the taxonomic
incommensurability thesis may be incorporated unproblematically within a reasonably robust
scientific realist framework" (Sankey 1998, 7).
"With this theory of kinds, Kuhn redraws the picture of scientific revolutions. Since the
interconnections among kind terms form a lexical taxonomy, scientific revolutions, which now are
limited to the meaning change of kind terms, become taxonomic changes. A scientific revolution
produces a new lexical taxonomy, in which some kind terms refer to new referents that overlap with
those denoted by some old kind terms. Therefore, incommensurability does not result merely from
translation failures of individual concepts. The prerequisite for full translatability between two
taxonomies is not shared features of individual concepts, but a shared lexical structure (Kuhn
1990b, p. 7). Scientists from rival paradigms face incommensurability because they construct
different lexical taxonomies and thereby classify the world in different ways" (Chen 1997, 260;
Kuhn 1990b refers to an unpublished manuscript).

14. The following quote exemplifies the complex pattern of different interests that may be at play
behind given classifications — in particular in the domain of arts: "The work of DiMaggio (1987)
has provided the theoretical foundation for much research on classification systems. His concept of
artistic classification systems offers a number of insights. First, the study of classification
systems needs to take into account both the consumption and production of art. According to
DiMaggio (1987: 441), artistic classification systems consist of 'the way that the work of artists
is divided up both in the heads and habits of consumers and by the institutions that bound the
production and distribution of separate genres' [italics in original]. On the one hand,
classification systems arise out of processes of social distinction, whereby consumers use cultural
objects to mark social boundaries. These 'ritual classifications' can thus be influenced by social
structural factors at the societal level — such as stratification systems, elite cohesion, social
and geographic mobility, etc. — that generate demand for cultural boundaries. On the other hand,
classification systems are also influenced and mediated by ‘classification processes' at the
production side. DiMaggio (1987) identifies commercial classifications (the classifications used by
commercial producers to market their products), administrative classifications (created and
enforced by the state), and professional classifications (classifications driven by the incentives
of artists to differentiate and mark boundaries). The study of classification systems thus needs to
be attentive to the extent to which the categorical demands of consumers and the categories used by
producers overlap, diverge, or mutually reinforce each other" (Venrooij and Schmutz 2015, 799).

15. Nobes and Stadler (2013, 575) examine the degree to which classification is determined by who
is classifying and they refer to Bloor's (1982, 268) support for the claim of Durkheim and Mauss
(1903 [2010]) that the classification of things reproduces a pattern of social arrangements more
than a pattern of the things. Durkheim and Mauss found that our classifications necessarily are
hierarchical because human social organization, from which our view of nature ultimately flows, is
itself hierarchical. This view has been strongly criticized as 'sociologism', but is, as we saw,
defended by Bloor (1982).

16. Marradi wrote: "The opposition between "natural" and "artificial" classification is a recurring
theme in the last two centuries. Cohen and Nagel have cogently argued that 'any division ...
according to some actual trait arbitrarily chosen is perfectly natural... [but it] may also be said
to be artificial, in the sense that we select the trait' (1934, 223). Yet, many scholars have
continued stressing "naturality" as a desirable property, by gradually re-interpreting it in terms
of significant relationships with other classifications (Hempel 1961, version reprinted 1965,
146-7; Kaplan 1964, 50), utility 'for a wider range of inductive generalizations' (Gilmour 1940,
466), 'systematic import' (Huxley 1940; Hempel 1952; Sandri 1969), links with theory (Hempel 1952;
Bunge 1967, 83), 'projectibility of discriminating concepts' (Sandri 1969, 99 ff.). In short, the
concept of "natural classification" has been transferred from the ontological to the
epistemological domain. However, as Tiryakian (1968, 177) has remarked, 'the reification of
typologies is a frequent temptation and pitfall'. In a typical manual of the neopositivist period
one can still read a statement as plain as 'A natural class is based on the fundamental character
of things' (Lenzel 1938, 32). And one may suspect that, if the epistemological coat of paint were
scraped off, quite substantial traces of rusty essentialism would loom through the still widespread
concern for "natural" classifications" (Marradi 1990, 149).

17. Jon Fjeldså also gave a speech at the University of Copenhagen on 26 March 2014: "Får vi snart
en 'endelig' fugleklassifikation?" (Do we soon get a "final" classification of birds?)

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WordNet 3.1. "Classification". http://wordnetweb.princeton.edu/perl/webwn?s=classification

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Appendix: A sample of definitions of classification (chronological)

The intention is to provide a comprehensive list of definitions of classification, and the idea is
to update the list when new definitions are discovered in the literature. The sources for such
definitions are surprisingly few and meager. The Oxford English dictionary provides a number of
quotes back to 1767 but misses important ones such as Darwin (1859) and Huxley (1869) (see also
below under 2010). Historisches Wörterbuch der Philosophie Bd. 1-13 (edited by Joachim Ritter) has
no article "Klassifikation" or "Taxonomie" (but does have on, for example, "Kategorie,
Kategorienlehre", "Ordnung" and "System, Systematik, Systematisch"). McKenna and Bell (1998, 11-33)
provide an overview of the history and theory of classification and state: "The word
'classification' was not part of the scientific literature until the last decades of the eighteenth
century. The first use of which we are aware occurs in a botanical paper by the Marquis de
Condorcet (1777: 35)". However, earlier uses are listed below.

Plato (c.370 BC)
"Socrates: That of dividing things again by classes, where the natural joints are, and not trying
to break any part, after the manner of a bad carver." (Plato c.370 BC, Phaedrus 265e)

Aristotle (350 BCE)
"A 'genus' is what is predicated in the category of essence of a number of things exhibiting
differences in kind. We should treat as predicates in the category of essence all such things as it
would be appropriate to mention in reply to the question, 'What is the object before you?'; as, for
example, in the case of man, if asked that question, it is appropriate to say 'He is an animal'"
(Aristotle 350 BCE, Book I, chap. 1, part 5).
Note: "[T]he logic of classifications, which remains, in this time, the Aristotelian logic,
receives practically no new development until the 18th century" (Parrochia 2016; electronic source,
no pages; see also Sutcliffe 1993).

Michel Adanson (1763, clvi)
 "...all parts and qualities, or properties and faculties of plants... barring not even one" ought
to be considered before attempting a classification.

David Cranz (1767, vol. 1, ix)
"I have described what belongs to this science, not according to the classifications
[Ger. Eintheilungen] and characteristics, which are generally adopted by, and needful for the
modern naturalists, but according to a certain affinity or likeness."

Carl Linnaeus (1767, 152)
"Natura Insectorum per plures eorum ætates jam perspecta, superest ut systematice eadem
contemplemur. Recta autem eorum CLASSIFICATIO vitam huic scientiæ & facultatem conciliat, ubi
singula insecta suum quasi nomen ipsa produnt"
= "The nature of insects through their several ages [stages of life] having already been examined,
it remains to contemplate them systematically. Now the correct CLASSIFICATION of them [sci.,
insects] furnishes life and means to this science, where individual insects themselves produce, as
it were, their own name." (majuscule in original)

Thomas Reid (1785, 191)
"Our ability to distinguish and give names to the different attributes belonging to a single thing
goes along with an ability to observe that many things have certain attributes in common while they
differ in others. This enables us to put the countless hordes of individuals into a limited number
of classes, which are called 'kinds' and 'sorts' — and in the scholastic language called 'genera'
and 'species'" (here quoted from Frické 2012, 25).

William Whewell (1840, vol. 1, xxxiii, XCV)
"The attempts at Natural Classification are of three sorts; according as they are made by the
process of blind trial, of general comparison, or of subordination of characters. The process of
Blind Trial professes to make its classes by attention to all the characters, but without
proceeding methodically. The process of General Comparison professes to enumerate all the
characters, and forms its classes by the majority. Neither of these methods can really be carried
into effect. The method of Subordination of Characters considers some characters as more
important than others; and this method gives more consistent results than the others. This method,
however, does not depend upon the Idea of Likeness only, but introduces the Idea of Organization or
Function" (italics in original).

John Stuart Mill (1843, vol. 2, Bk. IV, Ch. 7, 299-300)
"Classification, thus regarded, is a contrivance for the best possible ordering of the ideas of
objects in our minds; for causing the ideas to accompany or succeed one another in such a way as
shall give us the greatest command over our knowledge already acquired, and lead more directly to
the acquisition of more. The general problem of Classification, in reference to these purposes, may
be stated as follows: To provide that things shall be thought of in such groups, and those groups
in such an order, as will best conduce to the remembrance and to the ascertainment of their laws."

William Benjamin Carpenter (1847, I. §2)
"The object of all Classification ... [is] to bring together those beings which most resemble each
other and to separate those that differ."

Charles Darwin (1859, 420)
"... all true classification is genealogical ..."
(See also Mayr and Bock 2002; Padian 1999.)

Thomas Henry Huxley (1869, 1)
"By the classification of any series of objects is meant the actual, or ideal, arrangement together
of those which are like and the separation of those which are unlike, the purpose of this
arrangement being to facilitate the operation of the mind in clearly conceiving and retaining in
the memory, the characters of the objects in question."

Charles Ammi Cutter (1876, 10)
"Class, a collection of objects having characteristics in common".

Robert Adamson (1901, vol. 1, 185)
"The process of arranging the objects of some province of experience into kinds or groups,
characterized by the possession of common marks.
As ordinarily defined, it involves more than logical DIVISION (q.v.), the rules of which furnish
the minimal conditions of the process. In addition, classification takes into account (1) either
the specific purpose of the arrangement, or (2) the natural conjunctions of marks which are of most
importance. In either case, the aim of classification is to render possible the greatest number of
general propositions regarding the objects, and so to facilitate the complete and systematic survey
of them. The ideal of a classification that is not determined by special, human ends, as e.g. in
classification of occupations in a census return, is to copy in its systematic arrangement the real
order of interdependence in the things themselves. What is called 'artificial,' as opposed to
natural classification, differs in degree only, not in kind. Literature: MILL, Logic, Bk. IV.
chaps. vii, viii; VENN, Empirical Logic, chap. xxx; JEVONS, Princ. of Sci., chap. xxx. (R.A.)."

Ernest Cushing Richardson (1901, 1)
Classification is the "putting together of like things, or more fully described, it is the
arranging of things according to likeness and unlikeness. It may also be expressed as the sorting
and grouping of things. It is convenient sometimes, to speak of ‘likeness and unlikeness' but
really in classification it is 'likeness' which rules while 'unlikeness' is merely what is left
over when likeness has been defined. The 'putting together of like things' is therefore the fullest
and most exact form of the definition."

Charles Sanders Peirce (1902/1931, Vol. 1, § 231, p. 103)
"All classification, whether artificial or natural, is the arrangement of objects according to
ideas. A natural classification is the arrangement of them according to those ideas from which
their existence results. No greater merit can a taxonomist have than that of having his eyes open
to the ideas in nature; no more deplorable blindness can affect him than that of not seeing that
there are ideas in nature which determine the existence of objects."

Henry E. Bliss (1935, 3)
"In dealing with the multiplicity of particular things, actualities, and specific kinds, we find
that some are alike, in general characters and in specific characteristics; and we may consequently
relate them in a class, or classes, that is classify them."

Jason E. L. Farradane (1950, 83).
"Classification is a theory of the structure of knowledge, i.e. of the relations between different
parts of knowledge. No arbitrary method of grouping, however carefully applied, is true
classification. The problem is primarily epistemological. What is true knowledge, and what are true
relations between the parts of knowledge? It is essential to define these if the classification is
to be true and logically sound."

Jason E. L. Farradane (1950, 87).
"A classification indicates the relations between items of knowledge."

Jason E. L. Farradane (1952, 73-74).
"A classification represents a theory of the structure of knowledge, i.e. of the relations between
the different parts of knowledge. The basic problem is to determine what these relations are and
how they link the different concepts from our knowledge into a coherent structure. The arbitrary or
'deductive' subdivision of an assumed total of knowledge cannot give a true representation of these
relations, which do not consist only of groupings of a class and its members, or division of a
whole into its parts. It was shown that a classification must be constructed 'inductively', or
upwards, piecing together known fragments of relations."

Jesse H. Shera (1965, 120)
"Classification is the crystallization or formalizing of inferential thinking, born of sensory
perception, conditioned by the operation of the human brain, and shaped by human experience. It
lies at the foundation of all thought, but it is pragmatic and it is instrumental."

Jesse H. Shera (1965, 127)
 "He [the librarian] must appreciate classification, not as a tool, but as a discipline in which is
to be studied the reaction and response of a living mind to the record left by a distant and
usually unknown mind; a discipline that seeks to achieve a better understanding of the changing
patterns of thought and the points of contact at which they can be related to specific units of
recorded information."
(Drucker 2014 is, however, a work demonstrating that this demand is not specific to the librarian
but describes, for example, the biologist and sexologist Alfred Kinsey equally well.)

Shiyali Ramamrita Ranganathan (1967, Chapter CP, 77-78)
Ranganathan distinguished five senses of 'classification'.
1. "Division. (See Chap CC).
This is the primitive meaning of the term 'classification'. Even a child practises classification
in Sense 1 with its playthings. Even early man had practised it.
2. Assortment [grouping of things of the same sort]. (See Chap CD).
Classification in Sense 2 is inherent in Man. Perhaps it is a concomitant of the finiteness of the
speed of neural impulses in the human body. When the speed is finite, structure emerges. Wherever
there is a structure, sequences emerge. When sequence is helpful to the purpose at hand, it is
Classification. The sequence inevitable inside of the skin, so to speak, gets expressed
extraneurally also. To classify in Sense 2 is thus a neural necessity. Sharpness in thinking,
clarity in expression, unerringness in communication, expedition in response, and exactness in
service depend ultimately on helpful sequence or Classification in Sense 2. The work of
philosophers and of taxonomists in the field of classification is generally restricted to
Classification in Sense 2. (See Chap CL).
3. Classification in sense 2 plus Representing each entity by an ordinal number taken out of a
system of ordinal numbers, designed to mechanise the maintenance of the sequence,
 1. Either when an entity has to be replaced after having been taken out of its position;
 2. Or when a new entity has to be interpolated or extrapolated in the correct place in the
sequence.

This ordinal number is the Class Number. (See Chap CG and CM).
Classification in Sense 3 is usually practised by large business concerns having to handle a large
number of commodities. The Customs Authorities too use Classification in Sense 3 in their published
list of commodities liable to customs duty.
4. Classification in Sense 3 when complete assortment is made of an amplified universe — that is,
when the entities and the pseudo-entities arising in the process of successive assortment stand
arranged in one filiatory sequence, each with its Class Number. (See Chap CH and CK).
Classification in Sense 4 is not used very much. It is only classification in Sense 3 and Sense 5
that are frequently in demand.
5. Classification in Sense 4 with all the entities removed but only the pseudo-entities or classes
retained — each class having the number representing it. (See Chap CM).
It is classification in Sense 5 that is used,
 1. Either when the universe classified is infinite;
 2. Or when some of the entities are unknown and unknowable at any moment, even though the universe
classified is finite.

In particular, it is Classification in Sense 5 that is practised by the library profession.
It should be recalled that that in classification in Sense 5
 1. The individual entities do not figure in the complete assortment;
 2. Classes take the place of entities ; and thus,
 3. Each class including the Original Universe is a Class of Classes.

Classification in Sense 2 has only a Scheme for Classes associated with it. But Classification in
Sense 5 has a Scheme for Classification associated with it. We shall restrict the meaning of the
term 'Classification' to Classification in Sense 5" (italics in original).

Bonifatii Mikhailovich Kedrow (1975, vol. 1, 3)
"Der Klassifizierung der Wissenschaften bedeutet den Zusammenhang der Wissenschaften, der in ihrer
Stellung in einer bestimmten Reihenfolge oder in einem System entsprechend einigen allgemeinen
Grundsätzen zum Ausdruck kommt" (italics in original).
This can perhaps be generalized in this way: Classification of objects means the display of
connections between the objects in a certain order or in a system reflecting certain basic
principles. Kedrow found that the principle of historicism must govern all natural classifications
(see section 4.2c γ Genetic/historicist approaches to classification).

Georg Klaus (1976, 628-629)
"Klassifikation. Verfahren zur Unterteilung eine Klasse K von Dingen usw. in Teilklassen" (628).
"Die dialektisch-materialistische Einstellung zur Klassifikation besteht also nicht etwa darin,
dass sie im Gegensatz zur antidialektischen die Berechtigung und den Wert von Klassifikationen
bestritte, sondern darin, dass sie die These von der zeitlichen und strukturellen Relativität der
Klassifikationen vertritt, während die antidialektische Auffassung die Klassifikationen in jeder
Hinsicht als absolut betrachtet."

Phillip R. Sloan (1981, 68)
"The arrangement of objects or entities into groups or classes, usually on the basis of perceived
similarity and difference."

ISO 5127-6 (1988, 93)
A classification system is an "indexing language intended for a structured representation of
documents or data, through the use of indexes and corresponding terms, in order to allow systematic
access, resorting to an alphabetical index, if necessary" (here cited from Simões et al. 2016, 531;
note that this standard has been revised by ISO 5127:2001).

Lois Mai Chan (1994, 259)
Classification is "the multistage process of deciding on a property or characteristic of interest,
distinguishing things or objects that possesses that property from those which lack it, and
grouping things or objects that have the property or characteristic in common into a class. Other
essential aspects of classification are establishing relationships among classes and making
distinctions within classes to arrive at subclasses and finer divisions."

Francis L. Miksa (1994, 144)
"[Bibliographic] Classification is the activity of creating categories into which bibliographic
items of all kinds may be placed (i.e., the work of the classificationist) and also the activity
of identifying bibliographical items in terms of the categories already extant in a given system
(i.e., the work of the classifier). It encompasses systems for arranging items on the shelves of
libraries (sometimes called 'bibliothecal' classification), as well as systems for arranging the
surrogates of items in catalogs (sometimes called 'bibliographical' classification). It includes
classificatory systems based on all kinds of item characteristics (subject, form, author, citation,
size, etc.), in all forms of order (logical and systematic, alphabetical, faceted, etc.), with all
kinds of operating methods (pre- and post-coordinated, statistically based clustering and
identification, etc.), and differing in scope from the universal to the very narrow. Finally,
library classification embraces a wide range of purposes, although most often its chief purpose has
been to facilitate document retrieval."

Frederick Wilfrid Lancaster (1998, 17)
Classification is "sorting items into 'conceptual classes'" and "forming classes of objects on the
basis of their subject matter".

Geoffrey Bowker and Susan Leigh Star (1999, 10)
"A classification is a spatial, temporal, or spatio-temporal segmentation of the world. A
'classification system' is a set of boxes (metaphorical or literal) into which things can be placed
to perform some kind of work — bureaucratic or knowledge production."

Arlene G. Taylor (1999, 237)
"The placing of subjects into categories; in organizing of information, classification is the
process of determining where an information package fits into a given hierarchy and then assigning
the notation associated with the appropriate level of the hierarchy to the information package and
its surrogate."

Satija (2000, 222)
"Classification means to divide objects/entities (both abstract and concrete) on the basis of their
differences or, conversely, the grouping of entities on the basis of their similarities.
Classification is any process of dividing, sorting, grouping, arranging, ordering, ranking, mapping
and correlating."

Elaine Svenonius (2000, 10)
"Organization can take many forms. Its prototypical form is classification. Classification brings
like things together. In traditional classifications, like things are brought together with respect
to one or more specified attributes. Any number of attributes can be used to form classes of
documents embodying information, such as same size or color, same subject, or same author. However,
the most important attribute for a system whose objectives is to organize information is the
attribute of 'embodying the same work'. "

Hubert Feger (2001, 1966)
"Classification is the assignment of objects to classes".
Later on, this was expanded: "The fundamental goal of classification is to find structures common
to a group of objects, using properties to classify the objects into subgroups based on the
similarity of their properties" (Feger 2015, 805).

The Portuguese Language Dictionary of the Academy of Sciences (Academia das Ciências de Lisboa
2001, 837).
Classification is the "action of distributing in classes, by categories ... according to precise
criteria" (cited from Simões et al. 2016, 531).

Ernst Mayr and Walter Joseph Bock (2002)
"The logical consequence of the definition of class is that classification must be defined as the
ordering of diversity into classes of similar entities. And this has been traditionally the almost
universally accepted concept of classification. [...] A classification of organisms is based on the
shared possession of their diverse attributes. The units of similarity in a Darwinian
classification are called taxonomic characters that have the property of being homologous to one
another in the several entities or groups. The claim of a few modern authors that there is no
agreement on the definition of the word 'classification' is quite misleading. Actually, prior to
1950 there was virtually total unanimity on the usage (in classification) of the words
classification and class, as referring to the grouping of similar items.
A classification is defined as "The arrangement of entities in a hierarchical series of nested
classes, in which similar or related classes at one hierarchical level are combined comprehensively
into more inclusive classes at the next higher level" (176).
"Class — (in classification) A group or collection of entities (individuals), possessing attributes
or traits in common ('being similar'), a kind or sort, grouped together under a general or class
name.
Classification — The arrangement of similar entities (objects) in a hierarchical series of nested
classes, in which each more inclusive higher-level class is subdivided comprehensively into less
inclusive classes at the next lower level.
Darwinian classification — The ordered grouping of organisms into classes, according to their
similarities and consistent with their inferred evolutionary history.
Downward classification — Establishing groups by logical division.
Evolutionary classification — A classification that duly considers both evolutionary processes, the
ecological adaptiveness of evolutionary divergence (degree of difference) and the genealogy
(phylogeny) of the taxa. Basically equal to a Darwinian classification.
Hierarchical classification — The system of ranks that indicates the categorical level (level of
difference) of each taxon" (191).

Kjeld Schmidt and Ina Wagner (2004, 392)
"Classification, in turn, is a special practice of categorization, involving pre-established and
systematic systems of signs. That is, classification is a linguistic operation of applying a
classification scheme, i.e., an ordered set of signs that is pre-established according to (a) some
general principles and criteria of ordering and (b) some procedures of identification and naming.
In short, an act of classification is an application of a classification scheme. Classification
systems (such as thesauri) can thus be seen as instantiations of classification schemes.
[...] Classifications and categorizations are both convention-based practices and equally so. But
classifications are convention-based in a quite specific sense. In the case of categorization there
are no pre-established principles and criteria for determining the correctness of an act of
categorization. With acts of classification, however, such pre-established principles and criteria
exist, in that they specify relationships between items in terms of, for example, class/
membership, part/whole, composition, cause/effect, origin/fate, function, ownership, value/risk,
location, or state. Accordingly, an actor applying a classification scheme in a particular case can
be held accountable in terms of the principles, criteria, and procedures of the classification
scheme."

Dagobert Soergel (2004, 358)
"A classification is a structure that organizes concepts into a meaningful hierarchy, possibly in a
scheme of facets. The classification of living things is a taxonomy. (The term taxonomy is
increasingly used for any type of classification.) A classification is now often called an
ontology, particularly if it gives richer concept relationships" (bold in original omitted here).

Faria and Pericão (2008, 258)
Classification is a "group of ordered concepts, distributed systematically in classes, forming a
structure" and a "structuring of concepts into classes and subdivisions to express the existing
semantic relationships between them" (here cited from Simões et al. 2016, 531).

Brian Campbell Vickery (2008, 145-6)
"A classification is a hierarchical structure of symbolic terms. The symbols point to, or
represent, entities in the real or an imagined world. The structure may be used as a ‘‘finding
aid’’, or information retrieval tool—to pinpoint a particular sought entity."

Clare Beghtol (2010, 1045)
"To classify means to put things into meaningful groups. Things can be physical objects, ideas,
events, or anything else that human beings can perceive or imagine, and a meaningful group can be
formed using any characteristic or combination of characteristics of the things. Groups can be
considered to be permanent or they can be considered temporary responses to a need of the moment."

Ingetraut Dahlberg (2010, 2941)
"With this journal [International classification, 1974-1992, thereafter Knowledge organization],
'classification' was understood as a multi-meaning word that includes the following concepts:
 1. classification in the sense of 'classification system', i.e., a system of classes arranged in
hierarchical or faceted order;
 2. classification in the sense of classifying, i.e., establishing a system of classes;
 3. classification in the sense of classing, i.e., relating the classes of a classification system
to objects or subjects of reality; and
 4. classification in the sense of classification science, i.e., relating to this field of study
and its activities" (list typography added).

Oxford English Dictionary (update from 2010)
"classification, n.
Origin: A borrowing from Latin. Etymon: Latin classificatio.
Etymology: < post-classical Latin classificatio (1673 in a German source; 1767 in Linnaeus) >
classical Latin classis class n. + -ficātiōn- , -ficātiō suffix. Compare German Klassifikation
(1760 or earlier as †Classification), Swedish klassifikation (1740 as †classification), Danish
klassifikation (1748), French classification (1780), Italian classificazione (1796). Compare
slightly later classify v.
 1. The result of classifying; a systematic distribution, allocation, or arrangement of things in a
number of distinct classes, according to shared characteristics or perceived or deduced affinities.
Also: a system or method for classifying.
 2. The action of classifying or arranging in classes, according to shared characteristics or
perceived affinities; assignment to an appropriate class or classes.
 3. A category to which something is assigned; a class."

Adriano Veloso and Wagner Meira (2011, 9)
"In a classification problem, there is a set of input-output pairs (also referred to as instances
or examples) of the form z[i] = (x[i]; y[i]): Each input x[i] is a fixed-length record of the form
‹a[1], . . . ,a[l]›; where each ai is an attribute-value. Each output y[i] draws its value from a
discrete and finite set of possibilities y = {c[1], . . .,c[p]}, and indicates the class to which
z[i] belongs. Cases where y[i] = ? indicate that the correct class of z[i] is unknown. There is a
fixed but unknown conditional probability distribution P(y|x), that is, the relationship between
inputs and outputs is fixed but unknown"
"This formulation implies that the classification problem corresponds to the problem of function
approximation." (Veloso and Meira 2011, 11).

Daniel Parrochia and Pierre Neuville (2013, 21)
"Definition 1.9.1 We call 'classification' the operation consisting of sharing, distributing or
allocating objects in classes or groups which are, in general, less numerous than them. [10] This
is also the result of this operation. We want, as much as it is possible, this result to be
constant, i.e. the classification must remain stable for a little transformation [11] of data".
Note 10: "In the case of infinite classifications, this requirement, of course, must be weakened:
we may only want the (infinite) cardinal of the classification to be less than or equal to the
(infinite) cardinal of the set of objects to be classified."
Note 11: "The sense of it will have to become clearer."

WordNet (3.1) [downloaded 2016-05-21] defines four senses of the noun 'classification' of which
three are relevant for this entry:
  * "S: (n) categorization, categorisation, classification, compartmentalization,
compartmentalisation, assortment (the act of distributing things into classes or categories of the
same type)
  * S: (n) classification, categorization, categorisation (a group of people or things arranged by
class or category)
  * S: (n) classification, categorization, categorisation, sorting (the basic cognitive process of
arranging into classes or categories)".

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Version 1.8 (= Version 1.3 plus section 3.6 and quotations from Adams and Adams (1991), Peirce
1902, Shera (1951), Vandamme (2015) and Vickery (2008); version 1.0 published 2017-02-09, this
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This article (version 1.0) is also published in Knowledge Organization. How to cite it: Hjørland,
Birger. 2017. “Classification”. Knowledge Organization 44, no. 2: 97-128. Also available in ISKO
Encyclopedia of Knowledge Organization, eds. Birger Hjørland and Claudio Gnoli,
http://www.isko.org/cyclo/classification

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