On The Parts Of Animals   

not include nor presuppose that of house-building; and the same is
true of all other productions. So that it must necessarily be that the
elementary material exists for the sake of the homogeneous parts,
seeing that these are genetically posterior to it, just as the
heterogeneous parts are posterior genetically to them. For these
heterogeneous parts have reached the end and goal, having the third
degree of composition, in which degree generation or development often
attains its final term.
Animals, then, are composed of homogeneous parts, and are also
composed of heterogeneous parts. The former, however, exist for the
sake of the latter. For the active functions and operations of the
body are carried on by these; that is, by the heterogeneous parts,
such as the eye, the nostril, the whole face, the fingers, the hand,
and the whole arm. But inasmuch as there is a great variety in the
functions and motions not only of aggregate animals but also of the
individual organs, it is necessary that the substances out of which
these are composed shall present a diversity of properties. For some
purposes softness is advantageous, for others hardness; some parts
must be capable of extension, others of flexion. Such properties,
then, are distributed separately to the different homogeneous parts,
one being soft another hard, one fluid another solid, one viscous
another brittle; whereas each of the heterogeneous parts presents a
combination of multifarious properties. For the hand, to take an
example, requires one property to enable it to effect pressure, and
another and different property for simple prehension. For this reason
the active or executive parts of the body are compounded out of bones,
sinews, flesh, and the like, but not these latter out of the former.
So far, then, as has yet been stated, the relations between these two
orders of parts are determined by a final cause. We have, however, to
inquire whether necessity may not also have a share in the matter; and
it must be admitted that these mutual relations could not from the
very beginning have possibly been other than they are. For
heterogeneous parts can be made up out of homogeneous parts, either
from a plurality of them, or from a single one, as is the case with
some of the viscera which, varying in configuration, are yet, to speak
broadly, formed from a single homogeneous substance; but that
homogeneous substances should be formed out of a combination of
heterogeneous parts is clearly an impossibility. For these causes,
then, some parts of animals are simple and homogeneous, while others
are composite and heterogeneous; and dividing the parts into the
active or executive and the sensitive, each one of the former is, as
before said, heterogeneous, and each one of the latter homogeneous.
For it is in homogeneous parts alone that sensation can occur, as the
following considerations show.
Each sense is confined to a single order of sensibles, and its organ
must be such as to admit the action of that kind or order. But it is
only that which is endowed with a property in posse that is acted on
by that which has the like property in esse, so that the two are the
same in kind, and if the latter is single so also is the former. Thus
it is that while no physiologists ever dream of saying of the hand or
face or other such part that one is earth, another water, another
fire, they couple each separate sense-organ with a separate element,
asserting this one to be air and that other to be fire.
Sensation, then, is confined to the simple or homogeneous parts. But,
as might reasonably be expected, the organ of touch, though still
homogeneous, is yet the least simple of all the sense-organs. For
touch more than any other sense appears to be correlated to several
distinct kinds of objects, and to recognize more than one category of
contrasts, heat and cold, for instance, solidity and fluidity, and
other similar oppositions. Accordingly, the organ which deals with
these varied objects is of all the sense-organs the most corporeal,
being either the flesh, or the substance which in some animals takes
the place of flesh.
Now as there cannot possibly be an animal without sensation, it