n some way be united to
the object about which it operates, it follows of necessity that this
something extrinsic, which is the object of the soul's operation,
must be related to the soul in a twofold manner. First, inasmuch as
this something extrinsic has a natural aptitude to be united to the
soul, and to be by its likeness in the soul. In this way there are
two kinds of powers--namely, the "sensitive" in regard to the less
common object--the sensible body; and the "intellectual," in regard
to the most common object--universal being. Secondly, forasmuch as
the soul itself has an inclination and tendency to the something
extrinsic. And in this way there are again two kinds of powers in the
soul: one--the "appetitive"--in respect of which the soul is referred
to something extrinsic as to an end, which is first in the intention;
the other--the "locomotive" power--in respect of which the soul is
referred to something extrinsic as to the term of its operation and
movement; for every animal is moved for the purpose of realizing its
desires and intentions.

The modes of living are distinguished according to the degrees of
living things. There are some living things in which there exists only
vegetative power, as the plants. There are others in which with the
vegetative there exists also the sensitive, but not the locomotive
power; such as immovable animals, as shellfish. There are others which
besides this have locomotive powers, as perfect animals, which require
many things for their life, and consequently movement to seek
necessaries of life from a distance. And there are some living things
which with these have intellectual power--namely, men. But the
appetitive power does not constitute a degree of living things;
because wherever there is sense there is also appetite (De Anima ii,
3).

Thus the first two objections are hereby solved.

Reply Obj. 3: The "natural appetite" is that inclination which each
thing has, of its own nature, for something; wherefore by its natural
appetite each power desires something suitable to itself. But the
"animal appetite" results from the form apprehended; this sort of
appetite requires a special power of the soul--mere apprehension does
not suffice. For a thing is desired as it exists in its own nature,
whereas in the apprehensive power it exists not according to its own
nature, but according to its likeness. Whence it is clear that sight
desires naturally a visible object for the purp