Objection 1. It would seem that human law does not prescribe acts of all the virtues. For vicious acts are contrary to acts of virtue. But human law does not prohibit all vices, as stated above (Article 2). Therefore neither does it prescribe all acts of virtue.
Objection 2. Further, a virtuous act proceeds from a virtue. But virtue is the end of law; so that whatever is from a virtue, cannot come under a precept of law. Therefore human law does not prescribe all acts of virtue.
Objection 3. Further, law is ordained to the common good, as stated above (Question 90, Article 2). But some acts of virtue are ordained, not to the common good, but to private good. Therefore the law does not prescribe all acts of virtue.
On the contrary, The Philosopher says (Ethic. v, 1) that the law "prescribes the performance of the acts of a brave man... and the acts of the temperate man... and the acts of the meek man: and in like manner as regards the other virtues and vices, prescribing the former, forbidding the latter."
I answer that, The species of virtues are distinguished by their objects, as explained above (54, 2; 60, 1; 62, 2). Now all the objects of virtues can be referred either to the private good of an individual, or to the common good of the multitude: thus matters of fortitude may be achieved either for the safety of the state, or for upholding the rights of a friend, and in like manner with the other virtues. But law, as stated above (Question 90, Article 2) is ordained to the common good. Wherefore there is no virtue whose acts cannot be prescribed by the law. Nevertheless human law does not prescribe concerning all the acts of every virtue: but only in regard to those that are ordainable to the common good--either immediately, as when certain things are done directly for the common good--or mediately, as when a lawgiver prescribes certain things pertaining to good order, whereby the citizens are directed in the upholding of the common good of justice and peace.
Reply to Objection 1. Human law does not forbid all vicious acts, by the obligation of a precept, as neither does it prescribe all acts of virtue. But it forbids certain acts of each vice, just as it prescribes some acts of each virtue.
Reply to Objection 2. An act is said to be an act of virtue in two ways. First, from the fact that a man does something virtuous; thus the act of justice is to do what is right, and an act of fortitude is to do brave things: and in this way law prescribes certain acts of virtue. Secondly an act of virtue is when a man does a virtuous thing in a way in which a virtuous man does it. Such an act always proceeds from virtue: and it does not come under a precept of law, but is the end at which every lawgiver aims.
Reply to Objection 3. There is no virtue whose act is not ordainable to the common good, as stated above, either mediately or immediately.
Source: Summa Theologica (New Advent)