12. All of nature, therefore, is good, since the Creator of all nature is supremely good. But nature is not supremely and immutably good as is the Creator of it. Thus the good in created things can be diminished and augmented. For good to be diminished is evil; still, however much it is diminished, something must remain of its original nature as long as it exists at all. For no matter what kind or however insignificant a thing may be, the good which is its "nature" cannot be destroyed without the thing itself being destroyed. There is good reason, therefore, to praise an uncorrupted thing, and if it were indeed an incorruptible thing which could not be destroyed, it would doubtless be all the more worthy of praise. When, however, a thing is corrupted, its corruption is an evil because it is, by just so much, a privation of the good. Where there is no privation of the good, there is no evil. Where there is evil, there is a corresponding diminution of the good. As long, then, as a thing is being corrupted, there is good in it of which it is being deprived; and in this process, if something of its being remains that cannot be further corrupted, this will then be an incorruptible entity [natura incorruptibilis], and to this great good it will have come through the process of corruption. But even if the corruption is not arrested, it still does not cease having some good of which it cannot be further deprived. If, however, the corruption comes to be total and entire, there is no good left either, because it is no longer an entity at all. Wherefore corruption cannot consume the good without also consuming the thing itself. Every actual entity [natura] is therefore good; a greater good if it cannot be corrupted, a lesser good if it can be. Yet only the foolish and unknowing can deny that it is still good even when corrupted. Whenever a thing is consumed by corruption, not even the corruption remains, for it is nothing in itself, having no subsistent being in which to exist.
13. From this it follows that there is nothing to be called evil if there is nothing good. A good that wholly lacks an evil aspect is entirely good. Where there is some evil in a thing, its good is defective or defectible. Thus there can be no evil where there is no good. This leads us to a surprising conclusion: that, since every being, in so far as it is a being, is good, if we then say that a defective thing is bad, it would seem to mean that we are saying that what is evil is good, that only what is good is ever evil and that there is no evil apart from something good. This is because every actual entity is good [omnis natura bonum est.] Nothing evil exists in itself, but only as an evil aspect of some actual entity. Therefore, there can be nothing evil except something good. Absurd as this sounds, nevertheless the logical connections of the argument compel us to it as inevitable. At the same time, we must take warning lest we incur the prophetic judgment which reads: "Woe to those who call evil good and good evil: who call darkness light and light darkness; who call the bitter sweet and the sweet bitter."  Moreover the Lord himself saith: "An evil man brings forth evil out of the evil treasure of his heart."  What, then, is an evil man but an evil entity [natura mala], since man is an entity? Now, if a man is something good because he is an entity, what, then, is a bad man except an evil good? When, however, we distinguish between these two concepts, we find that the bad man is not bad because he is a man, nor is he good because he is wicked. Rather, he is a good entity in so far as he is a man, evil in so far as he is wicked. Therefore, if anyone says that simply to be a man is evil, or that to be a wicked man is good, he rightly falls under the prophetic judgment: "Woe to him who calls evil good and good evil." For this amounts to finding fault with God's work, because man is an entity of God's creation. It also means that we are praising the defects in this particular man because he is a wicked person. Thus, every entity, even if it is a defective one, in so far as it is an entity, is good. In so far as it is defective, it is evil.
14. Actually, then, in these two contraries we call evil and good, the rule of the logicians fails to apply.  No weather is both dark and bright at the same time; no food or drink is both sweet and sour at the same time; no body is, at the same time and place, both white and black, nor deformed and well-formed at the same time. This principle is found to apply in almost all disjunctions: two contraries cannot coexist in a single thing. Nevertheless, while no one maintains that good and evil are not contraries, they can not only coexist, but the evil cannot exist at all without the good, or in a thing that is not a good. On the other hand, the good can exist without evil. For a man or an angel could exist and yet not be wicked, whereas there cannot be wickedness except in a man or an angel. It is good to be a man, good to be an angel; but evil to be wicked. These two contraries are thus coexistent, so that if there were no good in what is evil, then the evil simply could not be, since it can have no mode in which to exist, nor any source from which corruption springs, unless it be something corruptible. Unless this something is good, it cannot be corrupted, because corruption is nothing more than the deprivation of the good. 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. Only by corrupting something good can corruption inflict injury.
15. But when we say that evil has its source in the good, do not suppose that this denies our Lord's judgment: "A good tree cannot bear evil fruit."  This cannot be, even as the Truth himself declareth: "Men do not gather grapes from thorns," since thorns cannot bear grapes. Nevertheless, from good soil we can see both vines and thorns spring up. Likewise, just as a bad tree does not grow good fruit, so also an evil will does not produce good deeds. From a human nature, which is good in itself, there can spring forth either a good or an evil will. There was no other place from whence evil could have arisen in the first place except from the nature-good in itself-of an angel or a man. This is what our Lord himself most clearly shows in the passage about the trees and the fruits, for he said: "Make the tree good and the fruits will be good, or make the tree bad and its fruits will be bad."  This is warning enough that bad fruit cannot grow on a good tree nor good fruit on a bad one. Yet from that same earth to which he was referring, both sorts of trees can grow.
 Isa. 5:20.
 Matt. 12:35.
 This refers to Aristotle's well-known principle of "the excluded middle."