The Enchiridion

Accordingly, in the case of these contraries which we call good and evil, the rule of thelogicians, that two contraries cannot be predicated at the same time of the same thing, does not hold. No weather is at the same time dark and bright: no food or drink is at the same time sweet and bitter: no body is at the same time and in the same place black and white: none is at the same time and in the same place deformed and beautiful. And this rule is found to hold in regard to many, indeed nearly all, contraries, that they cannot exist at the same time in any one thing.

But although no one can doubt that good and evil are contraries, not only can they exist at the same time, but evil cannot exist without good, or in anything that is not good. Good, however, can exist without evil. For a man or an angel can exist without being wicked; but nothing can be wicked except a man or an angel: and so far as he is a man or an angel, he is good; so far as he is wicked, he is an evil. And these two contraries are so far co-existent, that if good did not exist in what is evil, neither could evil exist; because corruption could not have either a place to dwell in, or a source to spring from, if there were nothing that could be corrupted; and nothing can be corrupted except what is good, for corruption is nothing else but the destruction of good.

From what is good, then, evils arose, and except in what is good they do not exist; nor was there any other source from which any evil nature could arise. For if there were, then, in so far as this was a being, it was certainly a good: and a being which was incorruptible would be a great good; and even one which was corruptible must be to some extent a good, for only by corrupting what was good in it could corruption do it harm.