Objection 1. It would seem that evil is a nature. For every genus is a nature. But evil is a genus; for the Philosopher says (Praedic. x) that "good and evil are not in a genus, but are genera of other things." Therefore evil is a nature.
Objection 2. Further, every difference which constitutes a species is a nature. But evil is a difference constituting a species of morality; for a bad habit differs in species from a good habit, as liberality from illiberality. Therefore evil signifies a nature.
Objection 3. Further, each extreme of two contraries is a nature. But evil and good are not opposed as privation and habit, but as contraries, as the Philosopher shows (Praedic. x) by the fact that between good and evil there is a medium, and from evil there can be a return to good. Therefore evil signifies a nature.
Objection 4. Further, what is not, acts not. But evil acts, for it corrupts good. Therefore evil is a being and a nature.
Objection 5. Further, nothing belongs to the perfection of the universe except what is a being and a nature. But evil belongs to the perfection of the universe of things; for Augustine says (Enchir. 10,11) that the "admirable beauty of the universe is made up of all things. In which even what is called evil, well ordered and in its place, is the eminent commendation of what is good." Therefore evil is a nature.
On the contrary, Dionysius says (Div. Nom. iv), "Evil is neither a being nor a good."
I answer that, One opposite is known through the other, as darkness is known through light. Hence also what evil is must be known from the nature of good. Now, we have said above that good is everything appetible; and thus, since every nature desires its own being and its own perfection, it must be said also that the being and the perfection of any nature is good. Hence it cannot be that evil signifies being, or any form or nature. Therefore it must be that by the name of evil is signified the absence of good. And this is what is meant by saying that "evil is neither a being nor a good." For since being, as such, is good, the absence of one implies the absence of the other.
Reply to Objection 1. Aristotle speaks there according to the opinion of Pythagoreans, who thought that evil was a kind of nature; and therefore they asserted the existence of the genus of good and evil. For Aristotle, especially in his logical works, brings forward examples that in his time were probable in the opinion of some philosophers. Or, it may be said that, as the Philosopher says (Metaph. iv, text 6), "the first kind of contrariety is habit and privation," as being verified in all contraries; since one contrary is always imperfect in relation to another, as black in relation to white, and bitter in relation to sweet. And in this way good and evil are said to be genera not simply, but in regard to contraries; because, as every form has the nature of good, so every privation, as such, has the nature of evil.
Reply to Objection 2. Good and evil are not constitutive differences except in morals, which receive their species from the end, which is the object of the will, the source of all morality. And because good has the nature of an end, therefore good and evil are specific differences in moral things; good in itself, but evil as the absence of the due end. Yet neither does the absence of the due end by itself constitute a moral species, except as it is joined to the undue end; just as we do not find the privation of the substantial form in natural things, unless it is joined to another form. Thus, therefore, the evil which is a constitutive difference in morals is a certain good joined to the privation of another good; as the end proposed by the intemperate man is not the privation of the good of reason, but the delight of sense without the order of reason. Hence evil is not a constitutive difference as such, but by reason of the good that is annexed.
Reply to Objection 3. This appears from the above. For the Philosopher speaks there of good and evil in morality. Because in that respect, between good and evil there is a medium, as good is considered as something rightly ordered, and evil as a thing not only out of right order, but also as injurious to another. Hence the Philosopher says (Ethic. iv, i) that a "prodigal man is foolish, but not evil." And from this evil in morality, there may be a return to good, but not from any sort of evil, for from blindness there is no return to sight, although blindness is an evil.
Reply to Objection 4. A thing is said to act in a threefold sense. In one way, formally, as when we say that whiteness makes white; and in that sense evil considered even as a privation is said to corrupt good, forasmuch as it is itself a corruption or privation of good. In another sense a thing is said to act effectively, as when a painter makes a wall white.
Thirdly, it is said in the sense of the final cause, as the end is said to effect by moving the efficient cause. But in these two ways evil does not effect anything of itself, that is, as a privation, but by virtue of the good annexed to it. For every action comes from some form; and everything which is desired as an end, is a perfection. And therefore, as Dionysius says (Div. Nom. iv): "Evil does not act, nor is it desired, except by virtue of some good joined to it: while of itself it is nothing definite, and beside the scope of our will and intention."
Reply to Objection 5. As was said above, the parts of the universe are ordered to each other, according as one acts on the other, and according as one is the end and exemplar of the other. But, as was said above, this can only happen to evil as joined to some good. Hence evil neither belongs to the perfection of the universe, nor does it come under the order of the same, except accidentally, that is, by reason of some good joined to it.
Source: Summa Theologica (New Advent)