It is clear that whatever is received into something is received according to the condition of the recipient. Now a thing is known in as far as its form is in the knower. But the intellect knows a thing in its nature absolutely: for instance, it knows a stone absolutely as a stone; and therefore the form of a stone absolutely, as to its proper formal idea, is in the intellect. Therefore the intellect itself is an absolute form, and not something composed of matter and form. For if the intellect were composed of matter and form, the forms of things would be received into it as individuals, and so it would only know the individual: just as it happens with the sensitive powers which receive forms in a corporeal organ; since matter is the principle by which forms are individualized.
We must assert that the intellectual principle which we call the human soul is incorruptible. For a thing may be corrupted in two ways—"per se," and accidentally. Now it is impossible for any substance to be generated or corrupted accidentally, that is, by the generation or corruption 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. 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.
