n of something else. For generation and
corruption belong to a thing, just as existence belongs to it, which
is acquired by generation and lost by corruption. Therefore, whatever
has existence _per se_ cannot be generated or corrupted except "per
se"; while things which do not subsist, such as accidents and
material forms, acquire existence or lose it through the generation
or corruption of composite things. Now it was shown above (AA. 2, 3)
that the souls of brutes are not self-subsistent, whereas the human
soul is; so that the souls of brutes are corrupted, when their bodies
are corrupted; while the human soul could not be corrupted unless it
were corrupted _per se._ This, indeed, is impossible, not only as
regards the human soul, but also as regards anything subsistent that
is a form alone. For it is clear that what belongs to a thing by
virtue of itself is inseparable from it; but existence belongs to a
form, which is an act, by virtue of itself. Wherefore matter acquires
actual existence as it acquires the form; while it is corrupted so
far as the form is separated from it. But it is impossible for a form
to be separated from itself; and therefore it is impossible for a
subsistent form to cease to exist.

Granted even that the soul is composed of matter and form, as some
pretend, we should nevertheless have to maintain that it is
incorruptible. For corruption is found only where there is
contrariety; since generation and corruption are from contraries and
into contraries. Wherefore the heavenly bodies, since they have no
matter subject to contrariety, are incorruptible. Now there can be no
contrariety in the intellectual soul; for it receives according to
the manner of its existence, and those things which it receives are
without contrariety; for the notions even of contraries are not
themselves contrary, since contraries belong to the same knowledge.
Therefore it is impossible for the intellectual soul to be
corruptible. Moreover we may take a sign of this from the fact that
everything naturally aspires to existence after its own manner. Now,
in things that have knowledge, desire ensues upon knowledge. The
senses indeed do not know existence, except under the conditions of
"here" and "now," whereas the intellect apprehends existence
absolutely, and for all time; so that everything that has an
intellect naturally desires always to exist. But a natural desire
cannot be in vain. Therefore every intellectual substanc