In the

first place we must look to the constituent parts of
animals. For it is in a way relative to these parts, first and
foremost, that animals in their entirety differ from one another:
either in the fact that some have this or that, while they have not
that or this; or by peculiarities of position or of arrangement; or by
the differences that have been previously mentioned, depending upon
diversity of form, or excess or defect in this or that particular,
on analogy, or on contrasts of the accidental qualities.

To begin with, we must take into consideration the parts of
Man. For, just as each nation is wont to reckon by that monetary
standard with which it is most familiar, so must we do in other
matters. And, of course, man is the animal with which we are all of us
the most familiar.