s the act of matter, not by an
accidental quality, but by its own essence; otherwise matter and form
would not make a thing substantially one, but only accidentally one.
Therefore a form cannot be without its own proper matter. But the
intellectual principle, since it is incorruptible, as was shown above
(Q. 75, A. 6), remains separate from the body, after the dissolution
of the body. Therefore the intellectual principle is not united to
the body as its form.

_On the contrary,_ According to the Philosopher, _Metaph._ viii (Did.
vii 2), difference is derived from the form. But the difference which
constitutes man is "rational," which is applied to man on account of
his intellectual principle. Therefore the intellectual principle is
the form of man.

_I answer that,_ We must assert that the intellect which is the
principle of intellectual operation is the form of the human body. For
that whereby primarily anything acts is a form of the thing to which
the act is to be attributed: for instance, that whereby a body is
primarily healed is health, and that whereby the soul knows primarily
is knowledge; hence health is a form of the body, and knowledge is a
form of the soul. The reason is because nothing acts except so far as
it is in act; wherefore a thing acts by that whereby it is in act. Now
it is clear that the first thing by which the body lives is the soul.
And as life appears through various operations in different degrees of
living things, that whereby we primarily perform each of all these
vital actions is the soul. For the soul is the primary principle of
our nourishment, sensation, and local movement; and likewise of our
understanding. Therefore this principle by which we primarily
understand, whether it be called the intellect or the intellectual
soul, is the form of the body. This is the demonstration used by
Aristotle (De Anima ii, 2).

But if anyone says that the intellectual soul is not the form of the
body he must first explain how it is that this action of
understanding is the action of this particular man; for each one is
conscious that it is himself who understands. Now an action may be
attributed to anyone in three ways, as is clear from the Philosopher
(Phys. v, 1); for a thing is said to move or act, either by virtue of
its whole self, for instance, as a physician heals; or by virtue of a
part, as a man sees by his eye; or through an accidental quality, as
when we say that something that is white bu