In my post of a few days ago I attempted to synthesize Thomas’s Augustinian privation theory of evil and his theology of the possible (the possible is what is able to imitate God) to say that evil is not a possibility but a kind of non-possibility, or a liability. Although I’m not aware of Aquinas explicitly drawing this conclusion, Anselm essentially does. William Courtenay (Capacity and Volition) writes:
The aspect of divine omniptence that concerned Anselm… was precisely God’s ability to do that which is evil, sinful, or unjust. Over against Damian’s list of undesirable things God cannot do, such as lie, perjure, and act unjustly, Anselm listed God’s inability to be corrupted, or to make false what is true. Under the last category of actions impossible for God Anselm included the ability to change the past, thus rejecting Damian’s principal thesis….
Anselm was troubled by the fact that statements about God’s incapacity for wrongdoing were often phrased in language that appeared to limit or compromise divine omnipotence. Since the linguistic opposite of “inability” is “ability,” there would appear to be a whole range of things to which God’s “omnipotence” does not extend, things that fall within the power of every human creature, as Boethius had noted.
Anselm first addressed the topic of god’s inability to do unGodlike things in his Proslogion (1077-78) and De casu diaboli (1085-90). He ignored Boethius’s Neoplatonic, metaphysical solution to the problem and substituted a linguistic approach. Using Boethius’s notion of antecedent capacity, Anselm distinguished within it expressions that describe an actual ability or power and imprecise expressions of common usage that appear to do the same but in actuality attribute to someone or something a liability or a power possessed and potentially exercised by another… Anselm reduced the problem to a syntax fallacy.
On account of this impropriety of speaking, it happens that frequently we say “a thing can” not because it can, but because something else can; and [we say that] a thing which can, cannot, because some other thing cannot, just as if I say: “a book is able to be written by me,” when a book can do nothing, althouh I can write a book. And when we say: “this man cannot be conquered by that man,” we understand nothing other than: “that man cannot conquer this man.”
Hence we say that God is not able to do anything contrary to himself or perverse, since he is so powerful in beatitude and justice (nay rather since beatitude and justice are not in him separate thing sbut rather one good, so that he is omnioptence in the simplest good), that nothing is able to harm the highest good. Therefore he is not able to be corrupted nor to lie.
Power in the strict sense for Anselm, just like the freedom of the will, is the ability to do the good. The ability to do evil is not ability in the strict sense of the term but rather liability. Certain statements that seem to imply ability, such as “I can lie,” actually imply a liability. The same would apply to expressions such as “I can sin” or “I can do what is unjust.” Since liabilities do not belong ot the divine nature, it is nonsense to suggest that God’s freedom and omnipotence are limited by this type of inability. The problem is one of language, not of ontology or ethics. (Courtenay, Capacity, 31-2)