Thomas Aquinas, Compendium of Theology

A necessary consequence of the foregoing doctrine is that the intellect whereby man understands is incorruptible. Every being acts in a way that is conformable to its existence. The intellect has an activity which it does not share with the body, as we have proved. This shows that it can act by itself. Hence it is a substance subsisting in its own being. But, as was pointed out above, intellectual substances are incorruptible. Accordingly the intellect whereby man understands is incorruptible.

Again, the proper subject of generation and corruption is matter. Hence a thing is immune to corruption to the extent that it is free from matter. Things composed of matter and form are per se corruptible; material forms are corruptible indirectly (per accidens), though not per se. Immaterial forms, which are above material conditions, are wholly incorruptible. The intellect by its very nature is elevated completely beyond matter, as its activity shows: we do not understand anything unless we separate it from matter. Consequently the intellect is by nature incorruptible.

Moreover, corruption cannot take place without contrariety; for nothing is corrupted except by its contrary. This is why the heavenly bodies, which do not admit of contrariety, are incorruptible. But all contrariety is far removed from the nature of the intellect, so much so that things which are contraries in themselves, are not contraries in the intellect. The intelligible aspect of contraries is one, inasmuch as one thing is understood in terms of another. Thus it is impossible for the intellect to be corruptible.