Objection 1. It would seem that the mode of virtue falls under the precept of the law. For the mode of virtue is that deeds of justice should be done justly, that deeds of fortitude should be done bravely, and in like manner as to the other virtues. But it is commanded (Deuteronomy 26:20) that "thou shalt follow justly after that which is just." Therefore the mode of virtue falls under the precept.
Objection 2. Further, that which belongs to the intention of the lawgiver comes chiefly under the precept. But the intention of the lawgiver is directed chiefly to make men virtuous, as stated in Ethic. ii: and it belongs to a virtuous man to act virtuously. Therefore the mode of virtue falls under the precept.
Objection 3. Further, the mode of virtue seems to consist properly in working willingly and with pleasure. But this falls under a precept of the Divine law, for it is written (Psalm 99:2): "Serve ye the Lord with gladness"; and (2 Corinthians 9:7): "Not with sadness or necessity: for God loveth a cheerful giver"; whereupon the gloss says: "Whatever ye do, do gladly; and then you will do it well; whereas if you do it sorrowfully, it is done in thee, not by thee." Therefore the mode of virtue falls under the precept of the law.
On the contrary, No man can act as a virtuous man acts unless he has the habit of virtue, as the Philosopher explains (Ethic. ii, 4; v, 8). Now whoever transgresses a precept of the law, deserves to be punished. Hence it would follow that a man who has not the habit of virtue, would deserve to be punished, whatever he does. But this is contrary to the intention of the law, which aims at leading man to virtue, by habituating him to good works. Therefore the mode of virtue does not fall under the precept.
I answer that, As stated above (90, 3, ad 2), a precept of law has compulsory power. Hence that on which the compulsion of the law is brought to bear, falls directly under the precept of the law. Now the law compels through fear of punishment, as stated in Ethic. x, 9, because that properly falls under the precept of the law, for which the penalty of the law is inflicted. But Divine law and human law are differently situated as to the appointment of penalties; since the penalty of the law is inflicted only for those things which come under the judgment of the lawgiver; for the law punishes in accordance with the verdict given. Now man, the framer of human law, is competent to judge only of outward acts; because "man seeth those things that appear," according to 1 Samuel 16:7: while God alone, the framer of the Divine law, is competent to judge of the inward movements of wills, according to Psalm 7:10: "The searcher of hearts and reins is God."
Accordingly, therefore, we must say that the mode of virtue is in some sort regarded both by human and by Divine law; in some respect it is regarded by the Divine, but not by the human law; and in another way, it is regarded neither by the human nor by the Divine law. Now the mode of virtue consists in three things, as the Philosopher states in Ethic. ii. The first is that man should act "knowingly": and this is subject to the judgment of both Divine and human law; because what a man does in ignorance, he does accidentally. Hence according to both human and Divine law, certain things are judged in respect of ignorance to be punishable or pardonable.
The second point is that a man should act "deliberately," i.e. "from choice, choosing that particular action for its own sake"; wherein a twofold internal movement is implied, of volition and of intention, about which we have spoken above (Q8, 12): and concerning these two, Divine law alone, and not human law, is competent to judge. For human law does not punish the man who wishes to slay, and slays not: whereas the Divine law does, according to Matthew 5:22: "Whosoever is angry with his brother, shall be in danger of the judgment."
The third point is that he should "act from a firm and immovable principle": which firmness belongs properly to a habit, and implies that the action proceeds from a rooted habit. In this respect, the mode of virtue does not fall under the precept either of Divine or of human law, since neither by man nor by God is he punished as breaking the law, who gives due honor to his parents and yet has not the habit of filial piety.
Reply to Objection 1. The mode of doing acts of justice, which falls under the precept, is that they be done in accordance with right; but not that they be done from the habit of justice.
Reply to Objection 2. The intention of the lawgiver is twofold. His aim, in the first place, is to lead men to something by the precepts of the law: and this is virtue. Secondly, his intention is brought to bear on the matter itself of the precept: and this is something leading or disposing to virtue, viz. an act of virtue. For the end of the precept and the matter of the precept are not the same: just as neither in other things is the end the same as that which conduces to the end.
Reply to Objection 3. That works of virtue should be done without sadness, falls under the precept of the Divine law; for whoever works with sadness works unwillingly. But to work with pleasure, i.e. joyfully or cheerfully, in one respect falls under the precept, viz. in so far as pleasure ensues from the love of God and one's neighbor (which love falls under the precept), and love causes pleasure: and in another respect does not fall under the precept, in so far as pleasure ensues from a habit; for "pleasure taken in a work proves the existence of a habit," as stated in Ethic. ii, 3. For an act may give pleasure either on account of its end, or through its proceeding from a becoming habit.
Source: Summa Theologica (New Advent)