I answer that, 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’; while things which do not subsist, such as accidents and material forms, acquire existence or lost it through the generation or corruption of composite things. Now it was shown above (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.
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.