This post is one that has been restored after the hacking. All original comments were lost.
Evil is the absence or lack of the good.
Chapter 39: That no evil can be in God
1 HENCE it is manifestly apparent that evil cannot be in God.
2 For being and goodness and all essential predicates have nothing besides themselves added to them, although that which is or the good may have something besides being or goodness: since nothing hinders the subject of one perfection being the subject of another besides; thus that which is a body may be white and sweet: while every nature is confined within the bounds of its essence, so that it admits of nothing extraneous within itself. Now God is goodness and not merely good, as we have proved above. Therefore nothing that is not goodness can be in Him: and consequently evil can nowise be in Him.
Notes This is partly an argument of grammar. His “while every nature is confined within the bounds of its essence, so that it admits of nothing extraneous within itself” is nearly a tautology, both grammatical and logical. Point is, accepting that God is goodness itself (as proved last time), it follows that goodness itself can contain no evil.
3 Moreover. As long as a thing remains, that which is contrary to its essence is altogether incompatible with that thing: thus irrationality or insensibility is incompatible with man unless he cease to be man. Now the divine essence is goodness itself, as we have proved. Therefore evil which is contrary to good can have no place in God unless He cease to be God: which is impossible, since He is eternal, as was proved above.
Notes The urge to turn “thus irrationality or insensibility is incompatible with man unless he cease to be man” into a joke is almost irresistible. But I’ll rise above my vulgar tendencies to note an important point: being rational and sensible is what separates man from brute. Being rational is part of man’s nature. Remove that nature [insert joke here] and what is left is not man. The rest of Aquinas’s argument follows logically.
4 Again. Since God is His own being, nothing can be said of Him by participation, as is clear from the argument given above. If, then, evil were predicated of Him, it would be a predicate not by participation, but by essence. But evil cannot be predicated of any thing in such a way as to be the essence of that thing: for it would lack being, which is a good, as we have shown above: and in evil there can be no extraneous admixture, as neither can there be in goodness. Therefore evil cannot be predicated of God.
Notes Not for the last time it is emphasized that evil is the absence or lack of the good. This must be kept ever firmly in mind.
5 Again. Evil is opposed to good. Now the notion of good consists in perfection: and therefore the notion of evil consists in imperfection. Now defect or imperfection cannot be in God, since He is universally perfect, as shown above. Therefore evil cannot be in God.
6 Further. A thing is perfect according as it is in act. Therefore it will be imperfect according as it is deficient in act. Therefore evil is either privation or includes privation. Now the subject of privation is a potentiality: and this cannot be in God, and consequently neither can evil.
Notes Don’t forget that we earlier learned that to have potentiality is to lack perfection. The example given was instantiations of circles. No real circle can achieve perfect circleness. It is deficient in being in act of a perfect circle.
7 Moreover. If good is what is desired by all, it follows that evil as such is shunned by every nature. Now that which is in a thing against the mode of its natural appetite is violent and unnatural. Therefore evil in a thing is violent and unnatural in so far as it is an evil to that thing, although in composite things it may be natural thereto in respect of some part. But God is not composite, nor can anything be violent or unnatural in Him, as shown above. Therefore evil cannot be in God.
Notes This argument is easy. But don’t forget a good for one thing can be an evil for another. It’s good for the lion to gut a gazelle, but not so good for the gazelle. The lion desires its good, but its good results in unhappy ruminants.
 Ch. xxxviii.
 Ch. xv.
 Ch. xxxviii.
 Ch. xxxvii.
 Ch. xxviii.
 Ch. xvi.
 1 Ethic. i.
 Chs. xviii., xix.