(CG 3/10). That the Cause of Evil is good (Book 3. God, the End of Creatures) (Summa Contra Gentiles) (Aquinas, Thomas, St.)

Evil cannot exist by itself, having no essence (Chap. VII): it must therefore be in some subject: but every subject, being a substance, is some good.

3. A thing is called evil because it does harm: that can only be because it does harm to good: for to do harm to evil is a good thing, since the undoing of evil is good. But it would not do harm to good, formally speaking, unless it were in good: thus blindness does harm to a man inasmuch as it is within him.

But since good and evil are opposites, and one opposite cannot be the subject of another, but expels it, it seems at first sight strange if good is said to be the subject of evil. But if the truth is sought out, we shall find nothing strange or awkward in this conclusion. Good is commonly predicated as being is predicated, since every being, as such, is good. It is not strange that not-being should be in being as its subject: for every privation is some not-being, and still its subject is a substance, which is a being. Still not-being has not for its subject the being that is its opposite: thus sight is not the subject in which blindness is, but the animal. So the subject in which evil is, is not the good opposed to it, for that is taken away by the evil, but some other good. Thus the subject of moral evil is natural good: while natural evil, which is a privation of form, is in matter, and matter is good, as also is potential being.