ose things
which live: for we call living things "animate," [*i.e. having a
soul] and those things which have no life, "inanimate." Now life is
shown principally by two actions, knowledge and movement. The
philosophers of old, not being able to rise above their imagination,
supposed that the principle of these actions was something corporeal:
for they asserted that only bodies were real things; and that what is
not corporeal is nothing: hence they maintained that the soul is
something corporeal. This opinion can be proved to be false in many
ways; but we shall make use of only one proof, based on universal and
certain principles, which shows clearly that the soul is not a body.

It is manifest that not every principle of vital action is a soul,
for then the eye would be a soul, as it is a principle of vision; and
the same might be applied to the other instruments of the soul: but
it is the _first_ principle of life, which we call the soul. Now,
though a body may be a principle of life, as the heart is a principle
of life in an animal, yet nothing corporeal can be the first
principle of life. For it is clear that to be a principle of life, or
to be a living thing, does not belong to a body as such; since, if
that were the case, every body would be a living thing, or a
principle of life. Therefore a body is competent to be a living thing
or even a principle of life, as "such" a body. Now that it is
actually such a body, it owes to some principle which is called its
act. Therefore the soul, which is the first principle of life, is not
a body, but the act of a body; thus heat, which is the principle of
calefaction, is not a body, but an act of a body.

Reply Obj. 1: As everything which is in motion must be moved by
something else, a process which cannot be prolonged indefinitely, we
must allow that not every mover is moved. For, since to be moved is
to pass from potentiality to actuality, the mover gives what it has
to the thing moved, inasmuch as it causes it to be in act. But, as
is shown in _Phys._ viii, 6, there is a mover which is altogether
immovable, and not moved either essentially, or accidentally; and
such a mover can cause an invariable movement. There is, however,
another kind of mover, which, though not moved essentially, is moved
accidentally; and for this reason it does not cause an invariable
movement; such a mover, is the soul. There is, again, another mover,
which is moved essentially--namely, the body.