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(i) a family of actual individuals x1, x2, x3 . . . ;
(ii) a family of properties F,C,M . . . , attributed to individuals;
(iii) an 'essentiality specification' for every individual, as to whether this property is or is not essential to it;
(iv) relations between properties (for instance, relations of entailment).
Given a W1 with two individuals x1 and x2 and three properties F,C,M, the sign + means that an individual has the corresponding property, the sign - means that it does not, and the parentheses mark the essential properties, as shown in Figure 8.7.
W1
F
C
M
x1
(+)
(+)
-
x2
+
+
(-)

Figure 8.7
Now let us imagine a W2 in which are individuals with properties as shown in Figure 8.8.
W2
F
C
M
y1
(+)
(+)
+
y2
+
-
(-)
y3
(+)
(-)
(+)

Figure 8.8
An individual in W2 is the potential variant of its prototype individual in W1 if they differ only in accidental properties (therefore y1 in W2 is a variant of x1 in W1 and y2 in W2 is a variant of x2 in W1).
An individual in W2 is a supernumerary in respect to the individuals in W1 if it differs from them also in essential properties (therefore y3Î W2 is supernumerary in respect to W1).
When a prototype in W1 has no more than one potential variant in W2, potential variance coincides with so-called transworld identity.
If we come back for a moment to the example given in 8.6.1, where my mother-in-law wonders what might have happened if I had never married her daughter, we see that she refers to a world structure of the sort shown in Figure 8.9, where M is the essential property of being married to her daughter and P is the accidental property of being the author of this paper.
On the contrary, when one wonders what might have happened if the author of this paper had never married, one refers back to a world structure of the sort shown in Figure 8.10.

 
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