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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 Obj. 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 Obj. 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