Descartes was a mathematician and natural philosopher
before he was a metaphysician. From about 1630
on, he envisioned a new, comprehensive science of
nature, the construction of and justification for which
became his primary aim. The Meditations was to
provide the promised metaphysical foundations for the
new system.
To understand the role of the Meditations in establishing
foundations for physics, we need to appreciate
the scope of Descartes’ physics. Physics today is far
removed from “the world of nature,” if under that
term we imagine mineral formations, plants, and
animals. The science of physics studies nature at very
small (subatomic) or very large (astronomical or
cosmic) scales. Other natural sciences, including chemistry
and biology, study things at or near the scale of
living things. The mental world is now often placed in
opposition to the physical and natural worlds;
psychology (the study of mental life) is only sometimes
called a natural science.
Chapter9
C h a p t e r 9
The new science
Physics,physiology,and the passions
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In Descartes’ day, “physics” or “natural philosophy” meant simply
“the science of nature.” “Nature” encompassed everything having a
nature or essence (at least on the Earth), including human beings and
human cognition. Aristotelian works on psychology (including De
anima, “On the Soul,” as well as works on dreams, memory, and the
senses) were classed within physics. Descartes conceived of physics in
this broad sense. He included animal and human physiology, and even
the Passions of the Soul (11:326), within physics. While he wasn’t clear
on whether the human mind considered by itself fell within physics, he
was clear that mind–body union and interaction were part of physics
or the science of nature. Although affirming mind–body substance
dualism, he did not take that position to imply that the embodied
mind is somehow unnatural, or supernatural, or beyond natural
science. It was not Descartes’ intent to exclude mind from nature
through his dualism.
All the same, Descartes’ philosophy did realign the relation
between mind and matter and offered a revolutionary conception of
matter itself (by contrast with Aristotelian philosophy). It reduced
matter to extension, leaving it no properties beyond the geometrical
modes of extension: size, shape, position, and motion. This reconception
changed the way living things could be conceived. Descartes
thought of plants and animals as machines, denying them the active
principles and cognitive powers found in Aristotelian physiology and
psychology. In Descartes’ mechanistic physiology, all bodily
processes are viewed as interactions between particles according to
the laws of motion. He extended the notion of lawful regularity to
mind–body interaction, positing a permanent relation between brain
states and the sensations, appetites, and emotions they produce in the
mind. For him, there was no conflict between dualism and
psychophysical laws.
In this chapter, we will examine Descartes’ physics from its foundations
as provided in the Meditations (and elaborated in the
Principles). The broad conception of physics permits us to see how
various Meditations provide foundations for physics that would not
be apparent under the narrower, present-day conception. We can
include among the “physical” topics not only the new concept of
matter from Meditations 2, 3, and 5, and God’s role in preserving
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that matter, but also the analysis of mental faculties from Meditation
2, and human physiology and mind–body interaction from
Meditation 6.
Descartes’ revolution in physics
The sixteenth and seventeenth centuries saw rapid intellectual change
in Europe. The Aristotelian philosophy – dominant from the thirteenth
through the sixteenth centuries – was replaced by new
philosophies, including new philosophies of nature. Many factors
contributed to this development. Aristotelianism itself had changed
over the centuries and was challenged by the revival of Platonic
philosophy in the fifteenth century. It survived the challenge,
although in the sixteenth century syntheses of Plato and Aristotle
became common. In medicine and physiology, Aristotelian viewpoints
had been combined with Galenic anatomy and physiology
(Galen was a second-century physician in Alexandria, Egypt). The
Italian anatomist Andreas Vesalius revivified the study of anatomy
through the publication of carefully prepared anatomical drawings
based on dissection of human corpses. In astronomy, Copernicus
combined the ancient standards of mathematical exactness with the
desire for a coherent account of planetary motions, arguing that the
Earth moves around the Sun (opposing the Earth-centered cosmos of
Aristotle and of Ptolemy, a second-century Egyptian). In optics or
the theory of vision, the geometrical, intromission theory of Ibn alHaytham,
an eleventh-century Islamic philosopher, was made
available in a sixteenth-century Latin translation, which spurred new
work. In natural philosophy proper, ancient Greek atomistic theories
of matter were revived and discussed.
Early in the seventeenth century, demand intensified for a “new
science” of nature, and for a new philosophy to frame it. Francis
Bacon called for a reformed science, based on greater attention to
direct observation of nature, including observations made by artisans
and skilled craftsmen. Galileo defended the Copernican system in
astronomy through arguments against the assumptions of
Aristotelian physics and Ptolemaic astronomy, and with his telescopic
observations of the moons of Jupiter (thus questioning Earth’s
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uniqueness as a body about which other bodies revolve). He developed
a new science of motion (including a law describing the
acceleration of freely falling bodies). Kepler developed Copernican
planetary astronomy and combined al-Haytham’s optical theory with
a new understanding of the internal anatomy of the eye. William
Harvey proposed that the blood is pumped through the body so that it
circulates several times per hour (as opposed to previous theories that
blood slowly oozes and does not circulate).
These key developments in the “Scientific Revolution”all occurred
prior to 1633, the year in which Descartes completed his World. Yet
none of the authors mentioned proposed anything to rival the
comprehensiveness of Descartes’ new vision of nature, as presented in
his World and Principles. The earlier innovators made important
theoretical proposals in single areas of science, or, in Bacon’s case,
proposed a new method intended to generate, at some point, a
comprehensive new theory of nature. Descartes, in addition to
making individual discoveries and offering a new method, proposed a
comprehensive new theory. He was the first to present a wide-ranging
new system of nature, and that system was developed and elaborated
by his followers for more than fifty years after his death before being
displaced by Newton’s physics.
Overview of Aristotelian physics
The radical nature of Descartes’ proposals is best understood against
the background of the prevailing Aristotelian physics. A common
Aristotelian position held that natural bodies are a composite of form
and matter, and that matter could not exist without form. The form of
a thing determined its nature or essence. Forms were principles of
growth and change; they literally made things “be what they are” by
directing the development and activity of matter toward an end
(hence away from any lack or “privation” of that end). Nature was
divided into a variety of kinds of substance, each with its characteristic
pattern of activity. All bodies were thought to be composed of
the four elements: earth, air, fire, and water. In these elements, undifferentiated
matter takes on the forms of the four basic qualities: hot,
cold, wet, and dry. Earth is cold and dry, air hot and wet, fire hot and
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dry, and water cold and wet. Other qualities, such as color and odor,
also exist as forms that can be transmitted from bodies to the sense
organs.
Higher-level natural kinds were divided into mineral, vegetable,
and animal. In complex or “mixed” bodies (mixtures of elements),
the four elements served as the “matter,” and a kind-specific form
gave each type of thing its characteristic pattern of activity. Thus,
crystals or metals derive their properties from their forms (of quartz,
gold, etc.). The oak tree grows as it does because its form (contained
in the acorn) directs that growth. Similarly for the various kinds of
animals, including the human animal. Each has a characteristic
form, introduced into the reproductive “matter” of the female by the
male during procreation, which directs its growth and activity.
Certain powers or activities of this form are similar across animal
species – all animals have nutritive, locomotive, and sensory powers.
The form of the human animal also possesses a rational power, as its
defining or essential power. The forms of all natural things direct
them toward their natural end, whether the center of the universe (in
the case of the earthy element), or knowledge and wisdom (in the
case of a human being, the rational animal). In this way, Aristotle’s
physics compared all natural processes to the biological process of
growth.
Aristotelian physics strictly divided the heavens from the Earth.
The natural tendency of the earthy element to seek the center of the
universe fixes the Earth in that location. Water has the same
tendency, although less strongly, and so collects on the surface of the
Earth. Air and fire have upward tendencies, the second stronger than
the first. These four elements are found in the region of change,
extending up to the sphere of the Moon (which acts as a bubble
around the Earth). The Moon, Sun, planets, and “fixed stars” (an
outermost bubble) are carried around the Earth, embedded in crystalline
spheres. These spheres are not composed of the four elements
but of a fifth element, the quintessence (“quint”means fifth), which is
unchanging. The region above the sphere of the Moon, including the
Sun and planets, was considered to be unchanging, with the heavenly
bodies revolving around the Earth in uniform circular motion (which
did not count as “change” or “alteration” in an Aristotelian sense).
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To account for the apparently irregular motions of the planets,
uniform circular motions had to be compounded by adding additional
spheres.
To gain acceptance for his own physics, Descartes needed to break
the hold of Aristotelian physics on common sense. Aristotelian
physics says that for a body to keep moving, force must be constantly
applied. This accords with much of our everyday experience.
Descartes’ laws of motion say that a body moving in a straight line
will continue in motion unless hindered. Aristotelian physics says that
earthy matter naturally moves toward the center of the universe (and
of the Earth). Descartes says that invisible subtle matter, swirling in a
vortex around the Earth, pushes objects downward. Aristotelian
physics says that each natural kind contains a substantial form that
serves to produce its characteristic activities, including the growth and
development of living things. Descartes says that the various natural
kinds differ only in the sizes, shapes, positions, and motions of their
particles, and that animal bodies are mere machines. Aristotelian
physics says that objects have the properties they appear to have, in the
manner in which they appear to have them – that color, odor, and so
on are real qualities encountered in sensory experience. Descartes says
that, in objects, these qualities are really configurations of corpuscles
that, in the case of color, induce spin in particles of light and ultimately
cause a color sensation in the mind.
Descartes sought acceptance for a (then) counter-intuitive picture
of nature. We may find it difficult to appreciate the full force of his
problem, since his vision of nature has been partly retained, and it
informs today’s common sense.
Descartes’ new system
In developing his new physics, Descartes drew upon the empirical
results and theoretical proposals of previous authors, including the
optical work of al-Haytham and Kepler, the astronomical arguments
of Copernicus and Galileo, the circulatory theory of Harvey, and the
revival of ancient atomism. But he moved beyond these results by
proposing a new conception of homogeneous extended matter
governed by a few laws of motion.
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Although Copernicus and Galileo had challenged ancient physics
and astronomy, neither offered a new system of physics to encompass
both heavens and Earth. Descartes’ physics did just that, by
appealing only to particles in motion to explain all phenomena
throughout the material world – including the formation of the Sun
and solar system, the revolution of the planets around the Sun
(carried along by a whirling vortex of subtle matter), and, in principle,
everything observable in the heavens and on Earth. Descartes
used his new conception of matter in framing comprehensive and
detailed theories to explain the known phenomena of light, heat, fire,
weight, magnetism, various minerals, and the physiology of living
things. His proposed explanations were often quite fanciful, such as
the explanation of magnetism by corkscrew-shaped particles flowing
out from the poles of the Earth, circulating north and south, and
entering the opposite pole while passing through the threaded channels
in any magnetic bodies encountered along the way – with leftand
right-hand threading accounting for opposite polarity
(8A:275–310). These explanations were unified in that they appealed
only to the properties of size, shape, position, and motion. The role
of Descartes’ metaphysics was to show that these are the only properties
of matter, and hence must frame all explanatory hypotheses
concerning the material world.
We saw in Chapter 1 that Descartes developed his comprehensive
new theory during 1629–33, as he composed his World. This period
began with the “metaphysical turn” of 1629–30, during which he
claimed to have discovered the foundations of physics while thinking
about God and the soul (1:144). We can appreciate more fully how
this might have happened by comparing two strategies Descartes
used in justifying his new philosophy of nature. When offering a
sample of his new physics in the Discourse and essays, he did not
present a metaphysical justification for its basic principles. He put
forward as a hypothesis that bodies are composed of particles having
only the properties of size, shape, position, and motion. The
Discourse argues that this corpuscularian hypothesis is confirmed by
the wide range of effects it could explain (6:76). In effect, it offers an
empirical argument from explanatory unity in support of the new
physics.
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During this same period, Descartes also promised a metaphysical
demonstration for the basic principles of his physics (6:76). In letters
from 1638, he further elaborated the empirical argument but refused
to reveal the metaphysical demonstration (1:563–4, 2:199–200). He
mentioned both types of argument in a letter to the French mathematician
J.B. Morin, comparing his explanations with those of the
Aristotelians:
Compare my assumptions with the assumptions of others.
Compare all their real qualities, their substantial forms, their
elements and countless other such things with my single assumption
that all bodies are composed of parts. This is something
which is visible to the naked eye in many cases and can be proved
by countless reasons in others. All that I add to this is that the
parts of certain kinds of bodies are of one shape rather than
another. This in turn is easy to demonstrate to those who agree
that bodies are composed of parts. Compare the deductions I
have made from my assumption – about vision, salt, winds,
clouds, snow, thunder, the rainbow, and so on – with what the
others have derived from their assumptions on the same topics. I
hope this will be enough to convince anyone unbiased that the
effects which I explain have no other causes than the ones from
which I have deduced them. Nonetheless, I intend to give a
demonstration of it in another place. [2:200]
On some occasions Descartes does not explicitly reject the forms and
qualities of Aristotelian physics but merely observes that they need
not be “mentioned” in his physics (6:239, 3:492). Here, he takes his
argument from comparative simplicity and unity of explanation to be
sufficient to convince an unbiased mind that various natural
phenomena “have no other causes” than particles of various shapes –
that is, that they do not have as causes substantial forms and real qualities.
Such an argument might not convince an Aristotelian who
doubted whether Descartes’ explanations really were more successful
overall. Recognizing that the argument was not a strict demonstration,
he continued to speak of a metaphysical demonstration, which
became the Meditations.
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Foundations for physics
Another clue from 1638 can help us to see how Descartes thought
contemplation of God and the soul could yield foundations for his
physics, if we draw on Chapters 3–8. In letters to Mersenne and the
Jesuit Vatier, he explained that he withheld the metaphysical demonstration
from the Discourse because he didn’t want to introduce
radical skepticism into a popular work. Such skepticism was needed
so that his readers might “withdraw the mind from the senses”
(1:350–1, 560). From Chapter 7 we know that he put forth the pure
intellect, devoid of sensory material, as the instrument for knowing
not only the soul and God but also the essence of material things. He
appealed to this same instrument to support his claim (Chapter 5)
that God preserves matter in existence from moment to moment
(according to the laws of motion).
On this interpretation, the metaphysical turn toward the pure intellect
provided Descartes with a direct argument for core principles of
his physics. Let us put this reading to work, considering first the foundations
of Descartes’ physics of the material world and then his
physics of mind–body union and interaction.
Real qualities, extension as essence
Descartes’ denial of real qualities may not seem particularly radical
now. Everyone who is acquainted with basic physics or introductory
psychology knows the modern analysis of color perception, which
explains color in objects using wavelengths of light (a distant relative
of Descartes’ spinning particles). But to Aristotelians and others in
his audience, the denial of real qualities would have seemed particularly
difficult to accept. Let us put ourselves in their shoes to see
why.
Aristotelian real qualities are given that name because in an
Aristotelian account the qualities we sense are direct representatives,
or instances, of a quality in the object. When we see a red tulip, the
real quality of redness is transmitted to our senses and received by
our sensory soul as a “form without matter.” The form of redness is
what makes the tulip red; this same form is expressed in the red we
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experience, in accordance with Aristotle’s principle that like knows
like. In between, the form is transmitted “without matter”through the
air, into the eye, and down the optic nerve (conceived as a hollow
tube). This received form accounts for the phenomenal red we experience
in perceiving the tulip.
The Aristotelian account has common-sense appeal because it says
that our visual experience reveals the actual or “real” qualities of
things. In some ways, however, the account as given by Aristotle was
incomplete. Especially, the notion of a form without matter transmitted
through a medium had to be filled out by medieval and early
modern Aristotelians. If the form of red makes the object red, why
doesn’t it turn the intervening air red when transmitted through it?
But the air between the tulip and us does not appear red; nor does the
eye turn red when we see a red thing. To explain these facts, mainstream
scholastic Aristotelians in the thirteenth to seventeenth
centuries taught that the form in the medium has a special kind of
diminished existence, called “intentional being,” and so they termed
the transmitted form itself an “intentional species.” The technical
term “intentional” conveyed two things: first, that the species of color
“tends toward,” “points to,” or “represents” the color in the object;
and, second, that the species of color in the air has diminished being,
does not exist as a full-blown quality, and so does not turn the air red.
In this way, they sought to square Aristotelian doctrine with the
observed facts.
In the Dioptrics, Descartes proclaimed that he had no use for the
“intentional species that exercise the imagination of the philosophers”(6:85*).
In his account, everything in the sensory process (up to
mind–body interaction) is purely mechanical. The quality of red in
the object, the transmission of light and color, and the effect of light
and color on the nervous system all reduce to the size, shape, and
motion of particles. On this view (which is similar to Galileo’s position),
color in the object consists simply in the geometrical features of
its surface, which cause it to put one or another spin on the spherical
particles of light. This spin is transmitted to the eye, where it affects
the retinal nerves in one way if the object is what we call “blue” and in
another way if “red” (6:91–2). These differing effects in the nervous
system and brain then cause differing sensations in the soul. Descartes
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did not deny that objects are colored; rather, he denied that color is a
real quality of the sort envisioned by the Aristotelians. Color in
objects is a purely mechanical property that affects the nervous
system, causing a sensation of red. The experienced red – the content
of the sensation – has an arbitrary but lawful relation (established by
God or nature) with the brain that causes it, and hence with the physical
property in objects (6:130–1, 7:81).
As Descartes saw things, he needed to overcome the natural
human prejudice in favor of the Aristotelian resemblance thesis (as
discussed in Chapters 5 and 8) in order to gain acceptance for his own
theory. His initial reason for doubting that sensations of color and
other qualities resemble something in objects was that they are
“obscure and confused” (7:43, 80, 83). To see how this observation
could challenge resemblance, we must ask: obscure and confused by
comparison with what? If we simply consider the sensory experience,
of a red ball, say, it hardly seems that our perception of the ball’s
redness is any more or less confused than our perception of its roundness.
Both seem equally “in focus,” phenomenally speaking. We must
therefore seek another standard, by comparison with which our
perception of the ball’s color is obscure and confused, whereas
perception of its shape is not.
Meditations 3–5 provide the needed standard. Descartes could
argue that sensory ideas of color are obscure and confused by
comparison with a clear and distinct (purely intellectual) perception
of shape. In Meditation 3, he observes that color and other qualities
are thought of “in a very confused and obscure way, to the extent that
I do not know whether they are true or false, that is, whether the ideas
I have of them are ideas of real things or of non-things” (7:43–4; see
also 7:83). This experience contrasts with the intellectual perception
of extension and its modes in Meditation 5, from which the meditator
concludes that material things “are capable of existing, insofar as they
are the subject matter of pure mathematics, since I perceive them
clearly and distinctly” (7:71). Ostensibly, shape and other geometrical
modes are clearly perceived to be potential properties of any possible
material thing, whereas color is not.
An argument for excluding real qualities might then go as follows.
Extension is the essence of matter. My sensory ideas of shape, size,
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position, and motion therefore present properties that things can
have. I may sometimes be mistaken about the precise sizes and shapes
of actually existing things, but the economy of the senses as bestowed
by God ensures that I will be right some of the time. By contrast, I do
not clearly and distinctly perceive color to be a possible property of
objects. Ideas of color arise only from the senses, but I should rely
only on my clear and distinct intellectual perceptions to tell me what
properties things can have. My sensory ideas of color do not meet that
standard. Hence bodies do not have the “real quality” color.
This argument draws on points from Meditations 3–6. But it has a
serious flaw. It can be interpreted as an argument from ignorance.
Perhaps color is a real quality, and the human intellect simply fails to
perceive it to be one. We know Descartes was unwilling to assert that
we know “all the properties” that are in mind or matter (7:220).
Maybe we simply can’t say whether color is a real quality or not.
This raises a question of interpretation. Does Descartes claim to
perceive that color is not a possible mode of extension, or does he
merely not claim to perceive that it is a mode of extension, as he
perceives the geometrical modes to be? Does our intellectual perception
exclude the real quality from matter, or is the status of color
simply unknown?
It seems clear that Descartes actually wanted to exclude real qualities
from bodies. But if his argument rests entirely on a direct
intellectual perception that the real quality color is not a possible
mode of extended substance, discussion ends there. The exclusion
would rely on an intellectual perception of the nature of matter, pure
and simple. This one-step argument offers little help if we don’t immediately
come to the same metaphysical insight ourselves. In Chapter 8,
we found Descartes appealing to a principle that could shore up his
argument – the “constitutive essence” principle, according to which
all the modes of a substance must be “perceived through”its principal
attribute. If we think of extension as the intelligible object of geometry
(as we are instructed to do), then it has no color or other sensory
properties. It has only extension, which can be divided into parts
having size, shape, position, and motion (7:63–4, 73–4). If we subsequently
try to think of color as a mode of extension, we find only
obscurity and confusion. Indeed, Descartes claimed in the Principles
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that “we cannot find any intelligible resemblance between the color
which we suppose to be in objects and that which we experience in our
sensation” (8A:34).
In the argument for mind–body distinctness, Descartes parried the
charge of arguing from ignorance by observing that he had clear and
distinct perceptions that both mind and body can exist as complete
things, on their own, without the other. In the present case, he could
not argue that color is “really distinct” from body, for that would
make color a substance – and he held that physical color in bodies
arises from the size and shape (modes) of their surfaces, and that experienced
color is a mode of mind (an idea or sensation). However, he
might (and in fact did) argue that we can conceive of extended matter
as existing without color, or the other so-called secondary qualities. In
Part 2 of the Principles, Descartes repeated the point from Meditation
6 that the senses are not for showing us “what really exists in things”
but for telling us what is beneficial or harmful to the mind–body
complex (8A:41). To know what really exists in things, we must “lay
aside the preconceived opinions acquired from the senses, and in this
connection make use of the intellect alone, carefully attending to the
ideas implanted in it by nature”(8A:42). He continued:
If we do this, we shall perceive that the nature of matter, or body
considered in general, consists not in its being something which is
hard or heavy or colored, or which affects the senses in any way,
but simply in its being something which is extended in length,
breadth, and depth.
Considering the quality of hardness, he argued that if we never felt
any bodies, hence never felt them as hard, they would not “thereby
lose their bodily nature.” He extended this thought experiment to the
other merely sensory qualities:
By the same reasoning it can be shown that weight, color, and all
other such qualities that are perceived by the senses as being in
corporeal matter, can be removed from it, while the matter itself
remains intact; it thus follows that its nature does not depend on
any of these qualities. [8A:42]
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As in the argument for mind–body distinctness, Descartes here
claims to perceive body as a complete being while explicitly denying it
the perceived quality of color. It is not that he simply doesn’t think
about whether color is in body. He asserts that it is not. Hence,
assuming he is thinking clearly and distinctly, he can claim to
perceive that body, possessing essentially only the geometrical modes
of extension, can exist as a complete being without color (as a real
quality). But if matter can be a complete being without color, and if
the properties that things have on their own must be instances of
their essence, then here is an argument for excluding color. This argument
is not explicit in the Meditations but may be latent in the
perception of the essence of matter (Meditation 5) and the discussion
of the respective roles of the senses and intellect in knowledge of
bodies (Meditation 6).
Descartes drew other physical conclusions from the theory that
extension is the essence of matter. Famously, he inferred that there
can be no space distinct from matter, hence that a vacuum is impossible
and the universe is a plenum (8A:49). There is no void, only
matter in motion. Motion does considerable work in Descartes’
physics. Let us therefore turn to the laws governing it.
Immutability, laws of motion
In Descartes’ metaphysical terminology, motion is a mode of extension.
But extension, as the essence of matter, does not specify what
laws, if any, motions follow, or even say what will happen when
moving bodies collide. Two extended things cannot interpenetrate or
coexist in the same place, so something must give when one body hits
another. But the bare concept of extension offers no hint about the
outcome. Descartes’ concept of extended matter does not include the
Newtonian notion of mass, with its implications for momentum and
transfer of force upon impact.
Descartes conceived extended matter as intrinsically inert. It
contains no activity, and it accrues no force in virtue of being in
motion. Rather, all force and activity must be referred back to God,
who preserves matter in existence from moment to moment, as in the
Third Meditation.
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The Third Meditation did not mention laws of motion, but it
provided the foundation for such laws in God’s preserving action. The
laws themselves were set out in the World and Principles. God
preserves the motions of bodies in accordance with laws governing
their interactions upon impact (Descartes did not recognize laws for
action at a distance). Both works portray the universe as possibly
evolving out of a soup of particles, created by God with a certain
quantity of motion (11:32–5, 8A:101; see also 6:42–4). This “quantity”
is defined as the product of the speed of bodies and their volume
of matter. (Descartes did not allow for matter of differing specific
gravities; he explained the density in ordinary objects by hypothesizing
that some are more porous than others.) He conceived of speed
as a scalar quantity, which means that no change in the quantity of
motion occurs when a body changes direction. He then sought to
derive the laws of motion from God’s immutability; in preserving the
universe from moment to moment, he preserves the same quantity of
motion as at creation.
Descartes purported to derive three laws from God’s immutability.
The first is “that each thing, as far as it is able, always continues in the
same state; and so when it is once moved, it always continues on”
(8A:62*). This law describes motion and rest as persisting states of
things. The second is “that all motion, in and of itself, is along a
straight line; therefore, those things that move in a circle always tend
to move away from the center of the circle they are describing”
(8A:63*). These two laws are similar to Newton’s law of inertia (his
first law of motion) but differ from it because Descartes did not treat
motion as a vector quantity (which would mean that changes in direction
alter the quantity of motion). The third law is “that one body, in
colliding with a stronger body, loses none of its own motion; but in
colliding with a weaker body, it loses as much of its motion as it transfers
to that weaker body”(8A:65*). This law is implausible on the face
of it, since it suggests that a snooker ball could never move the
slightly larger balls used in pool, no matter how hard it was driven.
Descartes tried to explain away such counter-examples by observing
that in our matter-filled environment, the larger body is surrounded
by the fluid of the air, allegedly making it easier to move (8A:70) –
although presumably both bodies are surrounded in this way, so it is
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hard to see what differential effect there would be on their ease of
motion.
Descartes offered as a ground for his first two laws “the
immutability and simplicity of the operation by which God preserves
motion in matter. For he always preserves the motion in the precise
form in which it is occurring at the very moment when he preserves it,
without taking any account of the motion which was occurring a little
while earlier” (8A:63–4). The source of natural rectilinear motion is
God’s preserving power. This yields another difference between
Descartes’ law of rectilinear persistence and Newton’s law of inertia.
Newton’s inertial law became explanatorily basic – a place where
explanation stops. In Newtonian physics, the continued motion of
bodies in a straight line no longer requires explanation, by contrast
with the Aristotelian scheme, in which continuing motion requires a
continuing cause. (Newton himself, at least for a time, regarded
continuing motion as the product of an inertial force, but later
Newtonians conceived of inertial motion as basic, without a continuing
force.)
Descartes considered this picture, in which God preserves particles
of matter from moment to moment according to their tendencies, to
be certified by his metaphysics. Nonetheless, it has its problems. It
may be objected that in a single instant a particle can have no
tendency to move, since for Descartes (8A:53) motion is simply transference
from one location to another (requiring finite time). The force
of this objection depends partly on whether Descartes conceived of
instants as merely vanishingly small or as dimensionless points. But it
may also depend on whether God does book-keeping on the “tendencies”
or directions of particles in motion. If God recreates the
universe from point-instant to point-instant, then the notion of a
continuous direction of motion would be entirely dependent on God’s
preserving action, not on any “tendency” internal to the moving body
itself.
Another problem concerns the coherence of bodies. If bodies are
composed of innumerable parts, how do these parts cohere to form a
single unit? Apparently, he held that bodies form units because their
parts are at rest in relation to one another (8A:71). In a moving body,
the common motion of its particles would keep them together. The
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quantity of motion of a body is the product of the quantity of matter
times speed. The quantity of matter is simply the volume of matter
traveling together. Any impact between macro-level bodies involves
the surface of one body, such as a cue ball, touching the surface of
another body, such as the eight ball. The volume of the whole cue ball
is used to determine how it will move the eight ball. But the balls are
units only because their particles move together. As the lead particles
make contact, their motion should be changed relative to the rest of
the ball, breaking the unity. Why shouldn’t a collision result in both
balls dissolving into one another, like colliding puffs of cigar smoke?
Perhaps because, as Descartes says, particles with irregular shapes
catch on one another and hold together (8A:144). But how can infinitely
divisible matter hold any shape at all? Presumably, because God
so conserves it.
A further problem is that the three laws themselves do not define a
determinate outcome for cases of impact. The third law says that one
body loses as much quantity of motion as another gains. But it does
not say how much each loses and gains. To address this question,
Descartes provided seven rules of impact in the Principles (8A:67–70).
Although these rules allegedly follow from the three laws, they are not
strictly derivable. Moreover, as Leibniz later observed, when represented
graphically they yield discontinuities when the sizes and speeds
of bodies are only slightly altered. These problems led Malebranche,
who generally tried to defend Descartes’ physics, to admit the difficulty
and reformulate the laws.
In practice, this last problem had little effect on Descartes’ physics,
for he did not refer to the rules of impact again in the Principles, and
he only rarely mentioned the laws of motion themselves (8A:108, 117,
144, 170, 194). The explanatory work-horse in Descartes’ physics was
the mechanistic interaction of particles in accordance with their
shapes and motions. This picture required that such interactions be
governed in a regular way, a requirement addressed by the above laws.
But in justifying various mechanistic models, Descartes appealed to
analogies with ordinary cases of bodily interaction, not to precisely
calculated exchanges of quantity of motion.
The significance of Descartes’ laws of motion lies more in their
overall conception than in any technical contribution to the analysis
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of impact. They offered the general vision of law-governed interactions
of matter everywhere in the universe. The first and second laws
described rectilinear motion as a natural state that does not diminish
of itself and will continue for ever if unimpeded by other bodies.
Although Galileo is sometimes credited with formulating a protoversion
of Newtonian inertia, he in fact treated circular motion along
the surface of the Earth as the “natural” state. He had no conception
of straight-line inertia. Although Descartes’ conception did not
involve vector quantities, it was the historical precursor to Newton’s
law of inertial motion.
Matter, innate ideas, and eternal truths
The essence of matter is extension, whose properties are understood
through geometry. The ideas of geometrical essences inhere (innately)
in the human intellect. Descartes asked some fundamental questions
about these essences: why are they what they are? That is, why is the
essence of the circle what it is, the essence of the triangle what it is, and
so on? Furthermore, how does the mind come to have its innate ideas
of those essences? The answers to these questions refer to the creative
power of God.
As mentioned in previous chapters, Descartes held that the
geometrical essences, as the essences of all created things, are free
creations of God (1:145–6, 149–53, 7:380, 432, 435–6). God did not
look to any standard or model, either independent of or internal to
himself, in creating the essences; he simply made it the case that the
radii of a circle are all equal to one another, and the angles of a
triangle are equal to two right angles. He could have created other
mathematical rules, so that the radii are not equal (1:152). Since our
minds use the truths that actually were created, we may be unable to
conceive such possibilities. (Descartes had no inkling of nonEuclidean
geometry.) Nonetheless, he held, the mathematical essences
(and others) are free creations of God’s will.
The doctrine that the so-called eternal truths are God’s free
creations might seem to threaten human knowledge. What if God
were to change them? On Descartes’ view, nothing about the current
set of essences required God to create them instead of others. If he
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reconsidered his act of creation, today’s geometrical truth might be
tomorrow’s falsehood.
In fact, Descartes held that his doctrine provided a secure basis for
human knowledge. On his theory, along with the eternal truths, God
created the material world and various minds, with their respective
essences, and he implanted innate knowledge of those essences in
human intellects. God therefore adjusted the human intellect to the
essence of matter (and, presumably, of mind). Moreover, since he is
immutable, the threat of changing essences does not arise. Truths
about essences remain “eternal truths” because once willed they are
fixed for ever by God’s immutability (1:149, 152).
In Descartes’ context, this doctrine might actually have improved
the theological palatability of his claim to know the first principles of
physics. In the metaphysical theology of mainstream scholastic
Aristotelians, knowledge of the essences of things implied an understanding
of the absolute limits on God’s creative power. These
Aristotelians held that the essences of created things, such as a rabbit
or an oak tree, depend on God for their existence. But that did not
imply to them that God freely chose the essences things have. On
their view, God could not create a rabbit that wasn’t an animal, or
that violated the essence of rabbithood in any other way. The
essences are therefore eternal, because they are grounded in God’s
(eternally determined) creative power. God understands the essences
by understanding what he can and cannot create. On this view, a
natural philosopher claiming to know the essences of things – and
especially claiming to know possible essences a priori, as did
Descartes – would be claiming to have fundamental knowledge of
God’s power. Descartes’ doctrine that the eternal truths are free
creations permits the natural philosopher to claim such knowledge of
essences without thereby claiming to comprehend the structure or
limits of God’s creative power. Since it was a tenet of Catholic
theology that God cannot be fully comprehended, in this way
Descartes could sidestep theological problems that might otherwise
arise for his claim to understand “completely” what the essence of
matter is. He could claim that the human mind’s innate ideas are
perfectly adjusted to the created world, not because the human mind
is able to grasp the limits of God’s power but because God has freely
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created the world with its essences and our minds with ideas of those
essences.
A mechanical philosophy
Descartes envisioned the world as a grand machine. This machine was
not filled with cogs and gear-wheels but with fluids and pressures,
spinning particles, and bits of irregularly shaped matter interacting to
produce the phenomena of nature. In the Meditations, this mechanistic
vision appears in the description of the human body as “a kind
of machine equipped with and made of bones, nerves, muscles, veins,
blood and skin” (7:84; also 7:229–30, 602). It was extensively developed
in the Principles (as, too, in the posthumously published World).
The terms “mechanism” and “mechanical” have several meanings
that might fit the concept of a mechanical philosophy. Beyond the
comparison with a machine (discussed below), the terms may imply
the blind following of laws without the intervention of undetermined
will or choice. In this sense, even a dualistic psychology can be mechanistic
if soul-substance is governed by laws. (Descartes proposed that
the human will follows the rule of always choosing the apparent truth
or apparent good, exercising the freedom of indifference only when
the intellect fails to present clear truth or goodness [7:432–3].) In
Descartes’ physics, both material nature and mind–body interaction
are governed by exceptionless regularities. His vision of nature was
mechanistic in this first sense. But, unlike Newtonian physics,
Descartes’ natural philosophy is not filled with derivations from
quantitative laws. His only published successes in fitting quantities to
empirical phenomena were the sine law of refraction (6:101) and the
work on the rainbow (6:336–43). We have seen that his theory of
impact was generally a failure in fitting quantitative laws to observed
phenomena.
Another aspect of a “mechanical” philosophy of nature is the
banishment of active principles, vital forces, and action at a distance
from natural processes. Descartes rejected the Aristotelian vegetative
and sensitive souls and their relatives, which previously had been
thought to govern (with a kind of implicit intelligence) the organic
processes of living things. Whereas many previous natural philosoB
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phers, including the English theorist William Gilbert, as well as
Kepler, accepted animistic theories of magnetic attraction,
comparing the pull of iron to magnet to the attraction between lovers,
Descartes offered his purely mechanical theory of subtle magnetic
fluid (tiny spirals) interacting with threaded channels in magnetic
bodies. The heat of fire was reduced to particles in motion, the action
of light to pressure in an ætherial medium, and so on. There is no
action at a distance; all material interaction is by direct contact.
“Mechanistic” can also mean non-purposeful, or without a
guiding teleology. Descartes is famous for rejecting final causes from
physics on the grounds that the human mind cannot hope to discern
God’s plan (5:185, 7:374–5, 8A:15–16, 80–1). (A “final cause” is the
purpose for which something comes into being or changes state.) The
explicit target of this rejection is the view that all of nature has been
organized for the benefit of humankind. Descartes’ cosmos was
populated by many suns with many planets; he felt it ludicrous for
human beings to suppose that the Sun and stars were created
expressly for their benefit (3:431). He also banished final causes from
the laws of motion. Aristotelians thought that earthy matter moves
toward the center of the universe as its end or final cause (although
without attributing awareness or knowledge to it, contrary to
Descartes’caricature [7:442]). Descartes admitted no such “ends”into
material interactions or bodily motions.
However, Descartes did not banish all teleological thinking from
natural philosophy. In describing the composite human being (mind
and body), he spoke of God or nature having arranged the rules of
mind–body interaction so that sensations tend toward the preservation
of the composite being (hence toward the health of the body
[7:80, 87]). This counts as invoking purpose, teleology, or final causes
in analyzing the mind–body relation and the functioning of the
senses. Similar teleology appears in his physiology, where he spoke of
the “functions” of the parts of the body, or what they “serve” to do
(7:374–5; 11:121, 154, 224).
A final sense of “mechanical” means machine-like, or pertaining
to machines. The most basic notion of machine in Descartes’ time
derived from the ancient science of mechanics, in which the lever was
regarded as a simple machine. Descartes invoked this notion in
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proudly proclaiming of his philosophy that, “like mechanics, it
considers shapes and sizes and motions” (1:420), and he composed a
brief treatise on mechanics (1:435–47). But his philosophy was
mechanical in a broader sense, in that he compared natural
phenomena and animal bodies to complex machines with interacting
parts. Many of Descartes’ mechanical explanations take the form of
analogies with effects observed in ordinary experience (8A:324–6).
They use analogies to characterize micromechanisms, which in turn
(purport to) explain the known phenomena of nature. Descartes
explained the properties of water by comparing its particles to eels,
the viscosity of oil by comparing its particles to branchy bushes that
can stick together, like tumbleweeds (1:423), and magnetism through
screw-shaped effluvia and threaded channels (8A:275). His grandest
comparison was between the human body and the hydraulically
driven automata found in the royal gardens of Europe in his day
(11:130–1). This comparison again evokes latent teleology, regarding
the design or function of the parts of the machine.
Mechanized body, embodied mind
Descartes’ mechanical philosophy rejected animism everywhere, save
for the human body (joined with a mind or soul), and perhaps the
world as a whole (where quantity of motion is preserved by God).
From the perspective of twentieth-century naturalism, this seemed
like two animisms too many. In that century, Descartes was accused of
putting a “ghost in the machine” of the human body.
Emphasis on Descartes’ dualism can mask the extent to which he
promoted a naturalistic, anti-vitalistic materialism concerning living
things. Aristotelians and other vitalists were “naturalists”about living
things inasmuch as they considered plants and animals to be part of
nature, and hence their powers and active principles to be natural. By
the standards of twentieth-century materialistic naturalism, however,
their list of natural powers was too liberal. On those standards, the
vegetative and sensitive powers of the Aristotelians, as also Descartes’
immaterial mind, were non-naturalistic. Despite its dualism,
Descartes’ philosophy promoted a materialistic naturalism toward
living things by holding that plants, animals, and the human body are
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nothing but machines. Indeed, he extended that sort of naturalism to
animal psychology generally and to much of human psychology,
which he thought could be explained through bodily mechanisms
alone, independent of mind.
Machine men
The most complete description of Descartes’ mechanistic physiology
is the Treatise on Man, although portions were discussed in the
Discourse, Dioptrics, Meditations, Principles, and Passions. The basic
vision is simple – human and animal bodies are machines that
respond to their environments through sensory stimulation, seek food
when they haven’t eaten, form material memories, and learn in
response to sensory stimulation. As he wrote in Meditation 6, one
may consider the human body “as a kind of machine equipped with
and made up of bones, nerves, muscles, veins, blood and skin in such a
way that, even if there were no mind in it, it would still perform all the
same movements as it now does in those cases where movement is not
under control of the will or, consequently, of the mind” (7:84).
Moreover, since he denied minds to animals, all of their behavior had
to be accounted for mechanistically. Much of human behavior (all
responses not affected by will, or requiring general intelligence) could,
he thought, be explained in the same way (see also 6:56–9).
Descartes thought of human and animal bodies as powered by a
“fire without light”that burns in the heart (11:202, 333). This fire heats
and expands blood as it enters the heart, acting like a boiler in a steam
engine. The blood exits the heart moving quickly, some proceeding to
the brain. There, at the base of the brain, the “animal spirits” – the
subtler and livelier parts of the blood – are filtered out and enter the
central cavity of the brain through the pineal gland. Some of the
spirits then proceed down the nerves (conceived as hollow tubes) to
the muscles; on entering a muscle they cause it to inflate like a balloon,
become taut, and contract. Muscle movement and hence behavior are
determined by which tubules the spirits enter (11:129–43, 170–97).
A sensory-motor loop controls the dispersal of the spirits. Sensory
nerves are filaments encased in the tubules. When a sense organ is
stimulated, the filament jiggles in a certain way, which causes it to tug
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