Evil is not knowable by itself, because it is of the essence of evil to be a privation of good; and thus it can neither be defined nor known except through good" (Sum. Theol., I, q. 14, art. 10, ad 4).
"Vinegar and oil," as Aeschylus says (Agam. 322-3), "poured into the same vessel, stand apart in unfriendly separation." But in the vessel of the mind contraries do not indeed blend, but stand together, and even call for one another's presence, as elements mutually complementary. Thus, though darkness excludes light, and good evil, the idea of darkness is complementary to that of light, and the idea of evil complementary to that of good. This difference between the ideal and the actual order, that in actuality, contraries are mutually exclusive, while as thoughts they are mutually complementary, I do not remember to have seen noticed elsewhere.
Of God and His Creatures: 1.71