Accordingly, mortal sin is incompatible with the infused virtues, but is consistent with acquired virtue: while venial sin is compatible with virtues, whether infused or acquired.
Reply Obj. 1: Sin is contrary to virtue, not by reason of itself, but by reason of its act. Hence sin is incompatible with the act, but not with the habit, of virtue.
Reply Obj. 2: Vice is directly contrary to virtue, even as sin to virtuous act: and so vice excludes virtue, just as sin excludes acts of virtue.
Reply Obj. 3: The natural powers act of necessity, and hence so long as the power is unimpaired, no sin can be found in the act. On the other hand, the virtues of the soul do not produce their acts of necessity; hence the comparison fails. ________________________
FIFTH ARTICLE [I-II, Q. 71, Art. 5]
Whether Every Sin Includes an Action?
Objection 1: It would seem that every sin includes an action. For as merit is compared with virtue, even so is sin compared with vice. Now there can be no merit without an action. Neither, therefore, can there be sin without action.
Obj. 2: Further, Augustine says (De Lib. Arb. iii, 18) [*Cf. De Vera Relig. xiv.]: So "true is it that every sin is voluntary, that, unless it be voluntary, it is no sin at all." Now nothing can be voluntary, save through an act of the will. Therefore every sin implies an act.
Obj. 3: Further, if sin could be without act, it would follow that a man sins as soon as he ceases doing what he ought. Now he who never does something that he ought to do, ceases continually doing what he ought. Therefore it would follow that he sins continually; and this is untrue. Therefore there is no sin without an act.
On the contrary, It is written (James 4:17): "To him . . . who knoweth to do good, and doth it not, to him it is a sin." Now "not to do" does not imply an act. Therefore sin can be without act.