is
the substantial form. Therefore the potentiality of matter is nothing
else but its essence.

Reply Obj. 3: Action belongs to the composite, as does existence;
for to act belongs to what exists. Now the composite has substantial
existence through the substantial form; and it operates by the power
which results from the substantial form. Hence an active accidental
form is to the substantial form of the agent (for instance, heat
compared to the form of fire) as the power of the soul is to the
soul.

Reply Obj. 4: That the accidental form is a principle of action is
due to the substantial form. Therefore the substantial form is the
first principle of action; but not the proximate principle. In this
sense the Philosopher says that "the soul is that whereby we
understand and sense."

Reply Obj. 5: If we take accident as meaning what is divided against
substance, then there can be no medium between substance and
accident; because they are divided by affirmation and negation, that
is, according to existence in a subject, and non-existence in a
subject. In this sense, as the power of the soul is not its essence,
it must be an accident; and it belongs to the second species of
accident, that of quality. But if we take accident as one of the five
universals, in this sense there is a medium between substance and
accident. For the substance is all that belongs to the essence of a
thing; whereas whatever is beyond the essence of a thing cannot be
called accident in this sense; but only what is not caused by the
essential principle of the species. For the 'proper' does not belong
to the essence of a thing, but is caused by the essential principles
of the species; wherefore it is a medium between the essence and
accident thus understood. In this sense the powers of the soul may be
said to be a medium between substance and accident, as being natural
properties of the soul. When Augustine says that knowledge and love
are not in the soul as accidents in a subject, this must be
understood in the sense given above, inasmuch as they are compared
to the soul, not as loving and knowing, but as loved and known. His
argument proceeds in this sense; for if love were in the soul loved
as in a subject, it would follow that an accident transcends its
subject, since even other things are loved through the soul.

Reply Obj. 6: Although the soul is not composed of matter and form,
yet it has an admixture of potentiality, as we have said above (