One objection to the Fourth Way is that it seems to be able to prove the existence of some greatest evil being. It’s not that St. Thomas didn’t see the objection, it’s that he was opposed – to the very core of his being – to ever treating a question outside of its proper place within a larger whole. When he finally gets to treating the question of the cause of evil in the Summa, he poses exactly the objection that the his impatient reader has been wondering about since he read the Fourth Way:
Objection 3. Further, as we find good and better things, so we find evil and worse. But good and better are so considered in relation to what is best. Therefore evil and worse are so considered in relation to some supreme evil.
The response would clearly appeal to evil as a privation, but the exact way in which STA does it is lovely:
Reply to Objection 3. Increase in intensity is in proportion to the nature of a thing. And as the form is a perfection, so privation removes a perfection. Hence every form, perfection, and good is intensified by approach to the perfect term; but privation and evil by receding from that term. Hence a thing is not said to be evil and worse, by reason of access to the supreme evil, in the same way as it is said to be good and better, by reason of access to the supreme good.
Note he is not denying the existence of a highest evil, but denying what it would mean to be the highest evil. He denies that a maximal evil is the proper measure of lesser evils, or if it is taken as a measure, that this measure is can exist or be intelligible apart from its relation to the supreme good.
(A second response would be to point out that there is a limit to how far evil can recede from a term. Given St. Thomas’s account of the infinite, “An infinite evil” would require that evil that could subsist without a subject – which is impossible for any privation.)
Just Thomism 