ttributed to man would belong
to the soul only; and whatever performs the operations proper to a
thing, is that thing; wherefore that which performs the operations of
a man is man. But it has been shown above (A. 3) that sensation is
not the operation of the soul only. Since, then, sensation is an
operation of man, but not proper to him, it is clear that man is not
a soul only, but something composed of soul and body. Plato, through
supposing that sensation was proper to the soul, could maintain man
to be a soul making use of the body.

Reply Obj. 1: According to the Philosopher (Ethic. ix, 8), a thing
seems to be chiefly what is princip[al] in it; thus what the governor
of a state does, the state is said to do. In this way sometimes what
is princip[al] in man is said to be man; sometimes, indeed, the
intellectual part which, in accordance with truth, is called the
"inward" man; and sometimes the sensitive part with the body is
called man in the opinion of those whose observation does not go
beyond the senses. And this is called the "outward" man.

Reply Obj. 2: Not every particular substance is a hypostasis or a
person, but that which has the complete nature of its species. Hence
a hand, or a foot, is not called a hypostasis, or a person; nor,
likewise, is the soul alone so called, since it is a part of the
human species.
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FIFTH ARTICLE [I, Q. 75, Art. 5]

Whether the Soul Is Composed of Matter and Form?

Objection 1: It would seem that the soul is composed of matter and
form. For potentiality is opposed to actuality. Now, whatsoever
things are in actuality participate of the First Act, which is God;
by participation of Whom, all things are good, are beings, and are
living things, as is clear from the teaching of Dionysius (Div. Nom.
v). Therefore whatsoever things are in potentiality participate of
the first potentiality. But the first potentiality is primary matter.
Therefore, since the human soul is, after a manner, in potentiality;
which appears from the fact that sometimes a man is potentially
understanding; it seems that the human soul must participate of
primary matter, as part of itself.

Obj. 2: Further, wherever the properties of matter are found, there
matter is. But the properties of matter are found in the
soul--namely, to be a subject, and to be changed, for it is a subject
to science, and virtue; and it changes from ignorance to knowledge
and from vice to virtue. Therefor