Objection 1. It would seem that prudence is not a distinct virtue from art. For art is the right reason about certain works. But diversity of works does not make a habit cease to be an art; since there are various arts about works widely different. Since therefore prudence is also right reason about works, it seems that it too should be reckoned a virtue.
Objection 2. Further, prudence has more in common with art than the speculative habits have; for they are both "about contingent matters that may be otherwise than they are" (Ethic. vi, 4,5). Now some speculative habits are called arts. Much more, therefore, should prudence be called an art.
Objection 3. Further, it belongs to prudence, "to be of good counsel" (Ethic. vi, 5). But counselling takes place in certain arts also, as stated in Ethic. iii, 3, e.g. in the arts of warfare, of seamanship, and of medicine. Therefore prudence is not distinct from art.
On the contrary, The Philosopher distinguishes prudence from art (Ethic. vi, 5).
I answer that, Where the nature of virtue differs, there is a different kind of virtue. Now it has been stated above (1; 56, 3) that some habits have the nature of virtue, through merely conferring aptness for a good work: while some habits are virtues, not only through conferring aptness for a good work, but also through conferring the use. But art confers the mere aptness for good work; since it does not regard the appetite; whereas prudence confers not only aptness for a good work, but also the use: for it regards the appetite, since it presupposes the rectitude thereof.
The reason for this difference is that art is the "right reason of things to be made"; whereas prudence is the "right reason of things to be done." Now "making" and "doing" differ, as stated in Metaph. ix, text. 16, in that "making" is an action passing into outward matter, e.g. "to build," "to saw," and so forth; whereas "doing" is an action abiding in the agent, e.g. "to see," "to will," and the like. Accordingly prudence stands in the same relation to such like human actions, consisting in the use of powers and habits, as art does to outward making: since each is the perfect reason about the things with which it is concerned. But perfection and rectitude of reason in speculative matters, depend on the principles from which reason argues; just as we have said above (2, ad 2) that science depends on and presupposes understanding, which is the habit of principles. Now in human acts the end is what the principles are in speculative matters, as stated in Ethic. vii, 8. Consequently, it is requisite for prudence, which is right reason about things to be done, that man be well disposed with regard to the ends: and this depends on the rectitude of his appetite. Wherefore, for prudence there is need of a moral virtue, which rectifies the appetite. On the other hand the good things made by art is not the good of man's appetite, but the good of those things themselves: wherefore art does not presuppose rectitude of the appetite. The consequence is that more praise is given to a craftsman who is at fault willingly, than to one who is unwillingly; whereas it is more contrary to prudence to sin willingly than unwillingly, since rectitude of the will is essential to prudence, but not to art. Accordingly it is evident that prudence is a virtue distinct from art.
Reply to Objection 1. The various kinds of things made by art are all external to man: hence they do not cause a different kind of virtue. But prudence is right reason about human acts themselves: hence it is a distinct kind of virtue, as stated above.
Reply to Objection 2. Prudence has more in common with art than a speculative habit has, if we consider their subject and matter: for they are both in the thinking part of the soul, and about things that may be otherwise than they are. But if we consider them as virtues, then art has more in common with the speculative habits, as is clear from what has been said.
Reply to Objection 3. Prudence is of good counsel about matters regarding man's entire life, and the end of human life. But in some arts there is counsel about matters concerning the ends proper to those arts. Hence some men, in so far as they are good counselors in matters of warfare, or seamanship, are said to be prudent officers or pilots, but not simply prudent: only those are simply prudent who give good counsel about all the concerns of life.
Source: Summa Theologica (New Advent)