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 (Q. 9, A. 1; Q. 17, AA. 1, 5; I, Q. 82, A. 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 Obj. 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 (Q. 25, AA. 1, 2, 3; Q. 27, A. 4; I, Q. 20, A. 1).

Reply Obj. 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.