CLE [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