Augustine on Evil

"Evils therefore, have their source in the good, and unless they are parasitic on something good, they are not anything at all. There is no other source whence an evil thing can come to be. If this is the case, then, in so far as a thing is an entity, it is unquestionably good. If it is an incorruptible entity, it is a great good. But even if it is a corruptible entity, it still has no mode of existence except as an aspect of something that is good."

Augustine, Enchiridion 14