(Hence also all contraries which are
different in species and not in genus are in the same line of
predication, and other than one another in the highest degree-for
the difference is complete-, and cannot be present along with one
another.) The difference, then, is a contrariety.
This, then, is what it is to be 'other in species'-to have a
contrariety, being in the same genus and being indivisible (and
those things are the same in species which have no contrariety,
being indivisible); we say 'being indivisible', for in the process
of division contrarieties arise in the intermediate stages before we
come to the indivisibles. Evidently, therefore, with reference to that
which is called the genus, none of the species-of-a-genus is either
the same as it or other than it in species (and this is fitting; for
the matter is indicated by negation, and the genus is the matter of
that of which it is called the genus, not in the sense in which we
speak of the genus or family of the Heraclidae, but in that in which
the genus is an element in a thing's nature), nor is it so with
reference to things which are not in the same genus, but it will
differ in genus from them, and in species from things in the same
genus. For a thing's difference from that from which it differs in
species must be a contrariety; and this belongs only to things in
the same genus.
9
One might raise the question, why woman does not differ from man
in species, when female and male are contrary and their difference
is a contrariety; and why a female and a male animal are not different
in species, though this difference belongs to animal in virtue of
its own nature, and not as paleness or darkness does; both 'female'
and 'male' belong to it qua animal.
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