The Enchiridion 14
14 Good and Evil are an Exception to the Rule that Contrary Attributes Cannot Be Predicated of the Same Subject. Evil Springs Up in What is Good, and Cannot Exist Except in What is Good PL 40, 238
Accordingly, in the case of these contraries which we call good and evil, the rule of the logicians, 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.
Bonum et malum, licet contraria, simul in eadem re esse posse. Ex bonis mala, et in bonis. Quapropter in iis contrariis quae mala et bona vocantur, illa dialecticorum regula deficit, qua dicunt nulli rei duo simul inesse contraria. Nullus enim aer simul est et tenebrosus et lucidus: nullus cibus aut potus simul dulcis et amarus: nullum corpus simul ubi album, ibi et nigrum; nullum simul ubi deforme, ibi et formosum. Et hoc in multis ac pene in omnibus contrariis reperitur, ut in una re simul esse non possint. Cum autem bona et mala nullus ambigat esse contraria, non solum simul esse possunt, sed mala omnino sine bonis et nisi in bonis esse non possunt: quamvis bona sine malis possint. Potest enim homo vel angelus non esse injustus; injustus autem non potest esse nisi homo vel angelus: et bonum quod homo, bonum quod angelus; malum quod injustus. Et haec duo contraria ita simul sunt, ut si bonum non esset in quo malum esset, prorsus nec malum esse potuisset: quia non modo ubi consisteret, sed unde oriretur corruptio non haberet, nisi esset quod corrumperetur; quod nisi bonum esset, nec corrumperetur; quoniam nihil est aliud corruptio, quam boni exterminatio. Ex bonis igitur mala orta sunt, et nisi in aliquibus bonis non sunt: nec erat alias unde oriretur ulla mali natura. Nam si esset, in quantum natura esset, profecto bona esset: et aut incorruptibilis natura magnum esset bonum, aut etiam natura corruptibilis nullo modo esset nisi aliquod bonum, quod bonum corrumpendo posset ei nocere corruptio.