Objection 1. It would seem that the intellect is not the subject of virtue. For Augustine says (De Moribus Eccl. xv) that all virtue is love. But the subject of love is not the intellect, but the appetitive power alone. Therefore no virtue is in the intellect.
Objection 2. Further, virtue is referred to good, as is clear from what has been said above (Question 55, Article 3). Now good is not the object of the intellect, but of the appetitive power. Therefore the subject of virtue is not the intellect, but the appetitive power.
Objection 3. Further, virtue is that "which makes its possessor good," as the Philosopher says (Ethic. ii, 6). But the habit which perfects the intellect does not make its possessor good: since a man is not said to be a good man on account of his science or his art. Therefore the intellect is not the subject of virtue.
On the contrary, The mind is chiefly called the intellect. But the subject of virtue is the mind, as is clear from the definition, above given, of virtue (55, 4). Therefore the intellect is the subject of virtue.
I answer that, As we have said above (Question 55, Article 3), a virtue is a habit by which we work well. Now a habit may be directed to a good act in two ways. First, in so far as by the habit a man acquires an aptness to a good act; for instance, by the habit of grammar man has the aptness to speak correctly. But grammar does not make a man always speak correctly: for a grammarian may be guilty of a barbarism or make a solecism: and the case is the same with other sciences and arts. Secondly, a habit may confer not only aptness to act, but also the right use of that aptness: for instance, justice not only gives man the prompt will to do just actions, but also makes him act justly.
And since good, and, in like manner, being, is said of a thing simply, in respect, not of what it is potentially, but of what it is actually: therefore from having habits of the latter sort, man is said simply to do good, and to be good; for instance, because he is just, or temperate; and in like manner as regards other such virtues. And since virtue is that "which makes its possessor good, and his work good likewise," these latter habits are called virtuous simply: because they make the work to be actually good, and the subject good simply. But the first kind of habits are not called virtues simply: because they do not make the work good except in regard to a certain aptness, nor do they make their possessor good simply. For through being gifted in science or art, a man is said to be good, not simply, but relatively; for instance, a good grammarian or a good smith. And for this reason science and art are often divided against virtue; while at other times they are called virtues (Ethic. vi, 2).
Hence the subject of a habit which is called a virtue in a relative sense, can be the intellect, and not only the practical intellect, but also the speculative, without any reference to the will: for thus the Philosopher (Ethic. vi, 3) holds that science, wisdom and understanding, and also art, are intellectual virtues. But the subject of a habit which is called a virtue simply, can only be the will, or some power in so far as it is moved by the will. And the reason of this is, that the will moves to their acts all those other powers that are in some way rational, as we have said above (9, 1; 17, A1,5; I, 82, 4): and therefore if man do well actually, this is because he has a good will. Therefore the virtue which makes a man to do well actually, and not merely to have the aptness to do well, must be either in the will itself; or in some power as moved by the will.
Now it happens that the intellect is moved by the will, just as are the other powers: for a man considers something actually, because he wills to do so. And therefore the intellect, in so far as it is subordinate to the will, can be the subject of virtue absolutely so called. And in this way the speculative intellect, or the reason, is the subject of Faith: for the intellect is moved by the command of the will to assent to what is of faith: for "no man believeth, unless he will" [Augustine: Tract. xxvi in Joan.]. But the practical intellect is the subject of prudence. For since prudence is the right reason of things to be done, it is a condition thereof that man be rightly disposed in regard to the principles of this reason of things to be done, that is in regard to their ends, to which man is rightly disposed by the rectitude of the will, just as to the principles of speculative truth he is rightly disposed by the natural light of the active intellect. And therefore as the subject of science, which is the right reason of speculative truths, is the speculative intellect in its relation to the active intellect, so the subject of prudence is the practical intellect in its relation to the right will.
Reply to Objection 1. The saying of Augustine is to be understood of virtue simply so called: not that every virtue is love simply: but that it depends in some way on love, in so far as it depends on the will, whose first movement consists in love, as we have said above (25, A1,2,3; 27, 4; I, 20, 1).
Reply to Objection 2. The good of each thing is its end: and therefore, as truth is the end of the intellect, so to know truth is the good act of the intellect. Whence the habit, which perfects the intellect in regard to the knowledge of truth, whether speculative or practical, is a virtue.
Reply to Objection 3. This objection considers virtue simply so called.
Source: Summa Theologica (New Advent)