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Objection 1: It would seem that the soul is a body. For the soul
is the moving principle of the body. Nor does it move unless moved.
First, because seemingly nothing can move unless it is itself moved,
since nothing gives what it has not; for instance, what is not hot
does not give heat. Secondly, because if there be anything that moves
and is not moved, it must be the cause of eternal, unchanging
movement, as we find proved Phys. viii, 6; and this does not
appear to be the case in the movement of an animal, which is caused by
the soul. Therefore the soul is a mover moved. But every mover moved
is a body. Therefore the soul is a body.
Objection 2: Further, all knowledge is caused by means of a
likeness. But there can be no likeness of a body to an incorporeal
thing. If, therefore, the soul were not a body, it could not have
knowledge of corporeal things.
Objection 3: Further, between the mover and the moved there must be
contact. But contact is only between bodies. Since, therefore, the
soul moves the body, it seems that the soul must be a body.
On the contrary, Augustine says (De Trin. vi, 6) that the soul
"is simple in comparison with the body, inasmuch as it does not occupy
space by its bulk."
I answer that, To seek the nature of the soul, we must premise that
the soul is defined as the first principle of life of those things which
live: for we call living things "animate", 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, or to be a living thing, 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 to Objection 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. And because the philosophers of old believed that nothing
existed but bodies, they maintained that every mover is moved; and
that the soul is moved directly, and is a body.
Reply to Objection 2: The likeness of a thing known is not of
necessity actually in the nature of the knower; but given a thing which
knows potentially, and afterwards knows actually, the likeness of the
thing known must be in the nature of the knower, not actually, but
only potentially; thus color is not actually in the pupil of the eye,
but only potentially. Hence it is necessary, not that the likeness of
corporeal things should be actually in the nature of the soul, but that
there be a potentiality in the soul for such a likeness. But the
ancient philosophers omitted to distinguish between actuality and
potentiality; and so they held that the soul must be a body in order to
have knowledge of a body; and that it must be composed of the
principles of which all bodies are formed in order to know all bodies.
Reply to Objection 3: There are two kinds of contact; of
"quantity," and of "power." By the former a body can be touched
only by a body; by the latter a body can be touched by an incorporeal
thing, which moves that body.
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