(A species is constituted by the combination differentia and matter.
For no part of an animal is purely material or purely immaterial;
nor can a body, independently of its condition, constitute an animal
or any of its parts, as has repeatedly been observed.)
Further, the differentiae must be elements of the essence, and not
merely essential attributes. Thus if Figure is the term to be divided,
it must not be divided into figures whose angles are equal to two
right angles, and figures whose angles are together greater than two
right angles. For it is only an attribute of a triangle and not part
of its essence that its angles are equal to two right angles.
Again, the bifurcations must be opposites, like White and Black,
Straight and Bent; and if we characterize one branch by either term,
we must characterize the other by its opposite, and not, for
example, characterize one branch by a colour, the other by a mode of
progression, swimming for instance.
Furthermore, living beings cannot be divided by the functions common
to body and soul, by Flying, for instance, and Walking, as we see them
divided in the dichotomies already referred to. For some groups,
Ants for instance, fall under both divisions, some ants flying while
others do not. Similarly as regards the division into Wild and Tame;
for it also would involve the disruption of a species into different
groups.
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