of the corporeal nature in various ways; for
the whole corporeal nature is subject to the soul, and is related to
it as its matter and instrument. There exists, therefore, an operation
of the soul which so far exceeds the corporeal nature that it is not
even performed by any corporeal organ; and such is the operation of
the _rational soul._ Below this, there is another operation of the
soul, which is indeed performed through a corporeal organ, but not
through a corporeal quality, and this is the operation of the
_sensitive soul;_ for though hot and cold, wet and dry, and other such
corporeal qualities are required for the work of the senses, yet they
are not required in such a way that the operation of the senses takes
place by virtue of such qualities; but only for the proper disposition
of the organ. The lowest of the operations of the soul is that which
is performed by a corporeal organ, and by virtue of a corporeal
quality. Yet this transcends the operation of the corporeal nature;
because the movements of bodies are caused by an extrinsic principle,
while these operations are from an intrinsic principle; for this is
common to all the operations of the soul; since every animate thing,
in some way, moves itself. Such is the operation of the _vegetative
soul;_ for digestion, and what follows, is caused instrumentally by
the action of heat, as the Philosopher says (De Anima ii, 4).

Now the powers of the soul are distinguished generically by their
objects. For the higher a power is, the more universal is the object
to which it extends, as we have said above (Q. 77, A. 3, ad 4). But
the object of the soul's operation may be considered in a triple
order. For in the soul there is a power the object of which is only
the body that is united to that soul; the powers of this genus are
called "vegetative" for the vegetative power acts only on the body to
which the soul is united. There is another genus in the powers of the
soul, which genus regards a more universal object--namely, every
sensible body, not only the body to which the soul is united. And
there is yet another genus in the powers of the soul, which genus
regards a still more universal object--namely, not only the sensible
body, but all being in universal. Wherefore it is evident that the
latter two genera of the soul's powers have an operation in regard
not merely to that which is united to them, but also to something
extrinsic. Now, since whatever operates must i