Objection 1. It would seem that moral virtue is a passion. Because the mean is of the same genus as the extremes. But moral virtue is a mean between two passions. Therefore moral virtue is a passion.
Objection 2. Further, virtue and vice, being contrary to one another, are in the same genus. But some passions are reckoned to be vices, such as envy and anger. Therefore some passions are virtues.
Objection 3. Further, pity is a passion, since it is sorrow for another's ills, as stated above (Question 35, Article 8). Now "Cicero the renowned orator did not hesitate to call pity a virtue," as Augustine states in De Civ. Dei ix, 5. Therefore a passion may be a moral virtue.
On the contrary, It is stated in Ethic. ii, 5 that "passions are neither virtues nor vices."
I answer that, Moral virtue cannot be a passion. This is clear for three reasons. First, because a passion is a movement of the sensitive appetite, as stated above (Question 22, Article 3): whereas moral virtue is not a movement, but rather a principle of the movement of the appetite, being a kind of habit. Secondly, because passions are not in themselves good or evil. For man's good or evil is something in reference to reason: wherefore the passions, considered in themselves, are referable both to good and evil, for as much as they may accord or disaccord with reason. Now nothing of this sort can be a virtue: since virtue is referable to good alone, as stated above (Question 55, Article 3). Thirdly, because, granted that some passions are, in some way, referable to good only, or to evil only; even then the movement of passion, as passion, begins in the appetite, and ends in the reason, since the appetite tends to conformity with reason. On the other hand, the movement of virtue is the reverse, for it begins in the reason and ends in the appetite, inasmuch as the latter is moved by reason. Hence the definition of moral virtue (Ethic. ii, 6) states that it is "a habit of choosing the mean appointed by reason as a prudent man would appoint it."
Reply to Objection 1. Virtue is a mean between passions, not by reason of its essence, but on account of its effect; because, to wit, it establishes the mean between passions.
Reply to Objection 2. If by vice we understand a habit of doing evil deeds, it is evident that no passion is a vice. But if vice is taken to mean sin which is a vicious act, nothing hinders a passion from being a vice, or, on the other hand, from concurring in an act of virtue; in so far as a passion is either opposed to reason or in accordance with reason.
Reply to Objection 3. Pity is said to be a virtue, i.e. an act of virtue, in so far as "that movement of the soul is obedient to reason"; viz. "when pity is bestowed without violating right, as when the poor are relieved, or the penitent forgiven," as Augustine says (De Civ. Dei ix, 5). But if by pity we understand a habit perfecting man so that he bestows pity reasonably, nothing hinders pity, in this sense, from being a virtue. The same applies to similar passions.
Source: Summa Theologica (New Advent)