I answer that, Where the nature of virtue differs, there is a different kind of virtue. Now it has been stated above (Article [1]; Question [56], Article [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 (Article [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.