Reply Obj. 2: The Philosopher is speaking there of a thing being against nature, in so far as "being against nature" is contrary to "being from nature": and not in so far as "being against nature" is contrary to "being in accord with nature," in which latter sense virtues are said to be in accord with nature, in as much as they incline us to that which is suitable to nature.
Reply Obj. 3: There is a twofold nature in man, rational nature, and the sensitive nature. And since it is through the operation of his senses that man accomplishes acts of reason, hence there are more who follow the inclinations of the sensitive nature, than who follow the order of reason: because more reach the beginning of a business than achieve its completion. Now the presence of vices and sins in man is owing to the fact that he follows the inclination of his sensitive nature against the order of his reason.
Reply Obj. 4: Whatever is irregular in a work of art, is unnatural to the art which produced that work. Now the eternal law is compared to the order of human reason, as art to a work of art. Therefore it amounts to the same that vice and sin are against the order of human reason, and that they are contrary to the eternal law. Hence Augustine says (De Lib. Arb. iii, 6) that "every nature, as such, is from God; and is a vicious nature, in so far as it fails from the Divine art whereby it was made." ________________________
THIRD ARTICLE [I-II, Q. 71, Art. 3]
Whether Vice Is Worse Than a Vicious Act?
Objection 1: It would seem that vice, i.e. a bad habit, is worse than a sin, i.e. a bad act. For, as the more lasting a good is, the better it is, so the longer an evil lasts, the worse it is. Now a vicious habit is more lasting than vicious acts, that pass forthwith. Therefore a vicious habit is worse than a vicious act.
Obj. 2: Further, several evils are more to be shunned than one. But a bad habit is virtually the cause of many bad acts. Therefore a vicious habit is worse than a vicious act.
Obj. 3: Further, a cause is more potent than its effect. But a habit produces its actions both as to their goodness and as to their badness. Therefore a habit is more potent than its act, both in goodness and in badness.
On the contrary, A man is justly punished for a vicious act; but not for a vicious habit, so long as no act ensues. Therefore a vicious action is worse than a vicious habit.
I answer that, A habit stands midway between power and act. Now it is evident that both in good and in evil, act precedes power, as stated in Metaph. ix, 19. For it is better to do well than to be able to do well, and in like manner, it is more blameworthy to do evil, than to be able to do evil: whence it also follows that both in goodness and in badness, habit stands midway between power and act, so that, to wit, even as a good or evil habit stands above the corresponding power in goodness or in badness, so does it stand below the corresponding act. This is also made clear from the fact that a habit is not called good or bad, save in so far as it induces to a good or bad act: wherefore a habit is called good or bad by reason of the goodness or badness of its act: so that an act surpasses its habit in goodness or badness, since "the cause of a thing being such, is yet more so."