Objection 1. It would seem that art is not an intellectual virtue. For Augustine says (De Lib. Arb. ii, 18,19) that "no one makes bad use of virtue." But one may make bad use of art: for a craftsman can work badly according to the knowledge of his art. Therefore art is not a virtue.
Objection 2. Further, there is no virtue of a virtue. But "there is a virtue of art," according to the Philosopher (Ethic. vi, 5). Therefore art is not a virtue.
Objection 3. Further, the liberal arts excel the mechanical arts. But just as the mechanical arts are practical, so the liberal arts are speculative. Therefore, if art were an intellectual virtue, it would have to be reckoned among the speculative virtues.
On the contrary, The Philosopher (Ethic. vi, 3,4) says that art is a virtue; and yet he does not reckon it among the speculative virtues, which, according to him, reside in the scientific part of the soul.
I answer that, Art is nothing else but "the right reason about certain works to be made." And yet the good of these things depends, not on man's appetitive faculty being affected in this or that way, but on the goodness of the work done. For a craftsman, as such, is commendable, not for the will with which he does a work, but for the quality of the work. Art, therefore, properly speaking, is an operative habit. And yet it has something in common with the speculative habits: since the quality of the object considered by the latter is a matter of concern to them also, but not how the human appetite may be affected towards that object. For as long as the geometrician demonstrates the truth, it matters not how his appetitive faculty may be affected, whether he be joyful or angry: even as neither does this matter in a craftsman, as we have observed. And so art has the nature of a virtue in the same way as the speculative habits, in so far, to wit, as neither art nor speculative habit makes a good work as regards the use of the habit, which is the property of a virtue that perfects the appetite, but only as regards the aptness to work well.
Reply to Objection 1. When anyone endowed with an art produces bad workmanship, this is not the work of that art, in fact it is contrary to the art: even as when a man lies, while knowing the truth, his words are not in accord with his knowledge, but contrary thereto. Wherefore, just as science has always a relation to good, as stated above (2, ad 3), so it is with art: and it is for this reason that it is called a virtue. And yet it falls short of being a perfect virtue, because it does not make its possessor to use it well; for which purpose something further is requisite: although there cannot be a good use without the art.
Reply to Objection 2. In order that man may make good use of the art he has, he needs a good will, which is perfected by moral virtue; and for this reason the Philosopher says that there is a virtue of art; namely, a moral virtue, in so far as the good use of art requires a moral virtue. For it is evident that a craftsman is inclined by justice, which rectifies his will, to do his work faithfully.
Reply to Objection 3. Even in speculative matters there is something by way of work: e.g. the making of a syllogism or of a fitting speech, or the work of counting or measuring. Hence whatever habits are ordained to such like works of the speculative reason, are, by a kind of comparison, called arts indeed, but "liberal" arts, in order to distinguish them from those arts that are ordained to works done by the body, which arts are, in a fashion, servile, inasmuch as the body is in servile subjection to the soul, and man, as regards his soul, is free [liber]. On the other hand, those sciences which are not ordained to any such like work, are called sciences simply, and not arts. Nor, if the liberal arts be more excellent, does it follow that the notion of art is more applicable to them.
Source: Summa Theologica (New Advent)