Reply. The reason why good and evil are reckoned specific differences in moral matters, is because moral matters depend on the will: for a thing enters into the sphere of morality inasmuch as it is voluntary. But the object of the will is the end in view and good: hence moral actions are specified by the end for which they are done, as physical actions are from the form of their active principle. Since then good and evil are designated according to due bearing on the end, or the lack of such due bearing, good and evil must be the first differentias marking species in moral matters. But the measure of morality is reason. Therefore things must be called morally good or evil according as they bear on the end which reason determines. Whatever therefore in moral matters derives its species from an end, which is according to reason, is said to be good in its species: while what derives its species from an end contrary to reason, is said to be evil in its species. But that end, though inconsistent with the end which reason assigns, is nevertheless some sort of good, as being pleasurable according to sense, or the like: hence such ends are good in some animals, and even in man when they are moderated by reason; and what is evil for one may very well be good for another. And therefore evil, inasmuch as it is a specific differentia in the genus of moral matters, does not involve anything that is essentially evil, but something that is in itself good, but evil to man inasmuch as it sets aside the order of reason, which is man's good.
Arg. 4. All that acts is something. But evil acts inasmuch as it is evil: for it understands good and spoils it. Evil therefore, inasmuch as it is evil, is some thing.
Reply. A privation, as such, is no principle of action. Hence it is well said that evil does not fight against good except in the power of good: but in itself it is impotent and weak and originative of no action. Evil is said however to spoil good also formally in itself, as blindness is said to spoil sight, or whiteness to colour a wall.
Arg. 5. Where there is found more and less, there must be an order of things, for negations and privations are not susceptible of more and less. But we find among evils one worse than another. Therefore evil must be some thing.
Reply. Conditions that imply privation are intensified or relaxed as are inequality and unlikeness: for a thing is more unequal according as it is further removed from inequality, and more unlike according as it is more removed from likeness: hence a thing is more evil according as it is a greater privation of good, or at a greater distance from good.*
Arg. 6. Thing and being are convertible terms. But evil is in the world. Therefore it is some thing and nature.
Reply. Evil is said 'to be' in the world, not as having any essence, or existing as a thing, but in the way in which a thing 'is' evil precisely by evil, as blindness, an in the way in which any privation is said 'to be,' inasmuch as an animal 'is' blind by blindness. For there are two senses of 'being': in one sense it means the essence of a thing, and is divided into the ten predicaments;* and in this sense no privation can be called a being: in another sense, it signifies the truth of an affirmative proposition (veritatem compositionis); and thus evil and privation is said to be a being, inasmuch as a thing is said to 'be' thereby under a privation.
3.7 : That Evil is not a Nature or Essence
3.10 : That the Cause of Evil is Good