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 (A. 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 of 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 Obj. 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 Obj. 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 Obj. 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. ________________________

FIFTH ARTICLE [I-II, Q. 57, Art. 5]