In the previous section, Thomas discussed the nature of the human intellect, and its division into the possible intellect, a kind of storehouse of things we’ve understood in the past, and the agent intellect, our capacity to actively bring these things to mind. Now he’s moving on from this to the incorruptibility of the human soul. He says,
A necessary consequence of the foregoing doctrine is that the intellect whereby man understands is incorruptible.
By “incorruptible,” Thomas means that the intellect cannot pass away, cannot die. Natural things are generated, they come to be, and they are corrupted, they pass away. They are born, and they die. The intellect does not. Thomas goes on to show this in several ways.
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.
As shown previously, the action of the intellect is immaterial; and since every being’s action is conformable with its existence, the intellect’s existence must be immaterial as well—it “subsists in its own being.” That is, it’s an intellectual substance, and such cannot die.
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.
Looked at another way, to be corruptible is to be material. A material object is composed of matter and form; the form gives form to the matter. An oak tree is matter with the form of an oak tree. Oak trees can die; and thus, indirectly, the tree’s form can die as well. Oak trees have no intellect, and no immaterial activity, and so the form must pass away with the matter.
But the intellect is an immaterial form, as it deals with forms as separated from matter and must be immaterial to do so.
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.
This last bit I had to think about for a while. “…nothing is corrupted except by its contrary.” What on earth does that mean?
Two propositions, A and B, are contraries if they cannot both be true. An oak tree exists; it dies; it no longer exists. There is no longer an oak tree. Those two statements are certainly contrary: there is an oak tree here, there is not an oak tree here. But I still think there’s something I’m missing about contraries and corruption.
But the point that Thomas is making about contraries is that although the propositions A and B cannot both be true in reality, and cannot coexist in fact, they can coexist perfectly well in my intellect. I am quite capable of holding both propositions in my mind at once—in fact, I must do so just to note that they are contraries. Thus, “contrariety is far removed from the nature of the intellect.” And since corruption requires contrariety, the intellect cannot be corruptible.