Thomas Aquinas, Compendium of Theology

Some may feel impelled to lodge a difficulty against this presentation: good cannot be the substratum of evil, for one of a pair of opposites cannot be the substratum of the other, nor do extremes ever exist together in other kinds of opposition. But let such quibblers reflect that other kinds of opposition belong to some definite genus, whereas good and evil are common to all genera. Every being, as such, is good; and every privation, as such, is evil. The substratum of a privation must be a being, hence good. But the subject of a privation need not be white or sweet or endowed with sight, because none of these predicates belongs to being as such. And so black is not in white, nor blindness in the person who sees; but evil is in good, just as blindness is in the sense that is the subject of sight. The reason why the subject of sight, in the case of a blind man, is not called "seeing," is that "seeing" is not a predicate common to every being.