Objection 1. It would seem that the irascible and concupiscible powers cannot be the subject of virtue. For these powers are common to us and dumb animals. But we are now speaking of virtue as proper to man, since for this reason it is called human virtue. It is therefore impossible for human virtue to be in the irascible and concupiscible powers which are parts of the sensitive appetite, as we have said in the I, 81, 2.
Objection 2. Further, the sensitive appetite is a power which makes use of a corporeal organ. But the good of virtue cannot be in man's body: for the Apostle says (Romans 7): "I know that good does not dwell in my flesh." Therefore the sensitive appetite cannot be the subject of virtue.
Objection 3. Further, Augustine proves (De Moribus Eccl. v) that virtue is not in the body but in the soul, for the reason that the body is ruled by the soul: wherefore it is entirely due to his soul that a man make good use of his body: "For instance, if my coachman, through obedience to my orders, guides well the horses which he is driving; this is all due to me." But just as the soul rules the body, so also does the reason rule the sensitive appetite. Therefore that the irascible and concupiscible powers are rightly ruled, is entirely due to the rational powers. Now "virtue is that by which we live rightly," as we have said above (Question 55, Article 4). Therefore virtue is not in the irascible and concupiscible powers, but only in the rational powers.
Objection 4. Further, "the principal act of moral virtue is choice" (Ethic. viii, 13). Now choice is not an act of the irascible and concupiscible powers, but of the rational power, as we have said above (Question 13, Article 2). Therefore moral virtue is not in the irascible and concupiscible powers, but in the reason.
On the contrary, Fortitude is assigned to the irascible power, and temperance to the concupiscible power. Whence the Philosopher (Ethic. iii, 10) says that "these virtues belong to the irrational part of the soul."
I answer that, The irascible and concupiscible powers can be considered in two ways. First, in themselves, in so far as they are parts of the sensitive appetite: and in this way they are not competent to be the subject of virtue. Secondly, they can be considered as participating in the reason, from the fact that they have a natural aptitude to obey reason. And thus the irascible or concupiscible power can be the subject of human virtue: for, in so far as it participates in the reason, it is the principle of a human act. And to these powers we must needs assign virtues.
For it is clear that there are some virtues in the irascible and concupiscible powers. Because an act, which proceeds from one power according as it is moved by another power, cannot be perfect, unless both powers be well disposed to the act: for instance, the act of a craftsman cannot be successful unless both the craftsman and his instrument be well disposed to act. Therefore in the matter of the operations of the irascible and concupiscible powers, according as they are moved by reason, there must needs be some habit perfecting in respect of acting well, not only the reason, but also the irascible and concupiscible powers. And since the good disposition of the power which moves through being moved, depends on its conformity with the power that moves it: therefore the virtue which is in the irascible and concupiscible powers is nothing else but a certain habitual conformity of these powers to reason.
Reply to Objection 1. The irascible and concupiscible powers considered in themselves, as parts of the sensitive appetite, are common to us and dumb animals. But in so far as they are rational by participation, and are obedient to the reason, they are proper to man. And in this way they can be the subject of human virtue.
Reply to Objection 2. Just as human flesh has not of itself the good of virtue, but is made the instrument of a virtuous act, inasmuch as being moved by reason, we "yield our members to serve justice"; so also, the irascible and concupiscible powers, of themselves indeed, have not the good of virtue, but rather the infection of the "fomes": whereas, inasmuch as they are in conformity with reason, the good of reason is begotten in them.
Reply to Objection 3. The body is ruled by the soul, and the irascible and concupiscible powers by the reason, but in different ways. For the body obeys the soul blindly without any contradiction, in those things in which it has a natural aptitude to be moved by the soul: whence the Philosopher says (Polit. i, 3) that the "soul rules the body with a despotic command" as the master rules his slave: wherefore the entire movement of the body is referred to the soul. For this reason virtue is not in the body, but in the soul. But the irascible and concupiscible powers do not obey the reason blindly; on the contrary, they have their own proper movements, by which, at times, they go against reason, whence the Philosopher says (Polit. i, 3) that the "reason rules the irascible and concupiscible powers by a political command" such as that by which free men are ruled, who have in some respects a will of their own. And for this reason also must there be some virtues in the irascible and concupiscible powers, by which these powers are well disposed to act.
Reply to Objection 4. In choice there are two things, namely, the intention of the end, and this belongs to the moral virtue; and the preferential choice of that which is unto the end, and this belongs to prudence (Ethic. vi, 2,5). But that the irascible and concupiscible powers have a right intention of the end in regard to the passions of the soul, is due to the good disposition of these powers. And therefore those moral virtues which are concerned with the passions are in the irascible and concupiscible powers, but prudence is in the reason.
Source: Summa Theologica (New Advent)