SECOND ARTICLE [I-II, Q. 71, Art. 2]
Whether Vice Is Contrary to Nature?
Objection 1: It would seem that vice is not contrary to nature. Because vice is contrary to virtue, as stated above (A. 1). Now virtue is in us, not by nature but by infusion or habituation, as stated above (Q. 63, AA. 1, 2, 3). Therefore vice is not contrary to nature.
Obj. 2: Further, it is impossible to become habituated to that which is contrary to nature: thus "a stone never becomes habituated to upward movement" (Ethic. ii, 1). But some men become habituated to vice. Therefore vice is not contrary to nature.
Obj. 3: Further, anything contrary to a nature, is not found in the greater number of individuals possessed of that nature. Now vice is found in the greater number of men; for it is written (Matt. 7:13): "Broad is the way that leadeth to destruction, and many there are who go in thereat." Therefore vice is not contrary to nature.
Obj. 4: Further, sin is compared to vice, as act to habit, as stated above (A. 1). Now sin is defined as "a word, deed, or desire, contrary to the Law of God," as Augustine shows (Contra Faust. xxii, 27). But the Law of God is above nature. Therefore we should say that vice is contrary to the Law, rather than to nature.
On the contrary, Augustine says (De Lib. Arb. iii, 13): "Every vice, simply because it is a vice, is contrary to nature."
I answer that, As stated above (A. 1), vice is contrary to virtue. Now the virtue of a thing consists in its being well disposed in a manner befitting its nature, as stated above (A. 1). Hence the vice of any thing consists in its being disposed in a manner not befitting its nature, and for this reason is that thing "vituperated," which word is derived from "vice" according to Augustine (De Lib. Arb. iii, 14).
But it must be observed that the nature of a thing is chiefly the form from which that thing derives its species. Now man derives his species from his rational soul: and consequently whatever is contrary to the order of reason is, properly speaking, contrary to the nature of man, as man; while whatever is in accord with reason, is in accord with the nature of man, as man. Now "man's good is to be in accord with reason, and his evil is to be against reason," as Dionysius states (Div. Nom. iv). Therefore human virtue, which makes a man good, and his work good, is in accord with man's nature, for as much as it accords with his reason: while vice is contrary to man's nature, in so far as it is contrary to the order of reason.
Reply Obj. 1: Although the virtues are not caused by nature as regards their perfection of being, yet they incline us to that which accords with reason, i.e. with the order of reason. For Cicero says (De Inv. Rhet. ii) that "virtue is a habit in accord with reason, like a second nature": and it is in this sense that virtue is said to be in accord with nature, and on the other hand that vice is contrary to nature.