Objection 1. It would seem that continence is not a virtue. For species and genus are not co-ordinate members of the same division. But continence is co-ordinated with virtue, according to the Philosopher (Ethic. vii, 1,9). Therefore continence is not a virtue.
Objection 2. Further, no one sins by using a virtue, since, according to Augustine (De Lib. Arb. ii, 18,19), "a virtue is a thing that no one makes ill use of." Yet one may sin by containing oneself: for instance, if one desire to do a good, and contain oneself from doing it. Therefore continence is not a virtue.
Objection 3. Further, no virtue withdraws man from that which is lawful, but only from unlawful things: for a gloss on Galatians 5:23, "Faith, modesty," etc., says that by continence a man refrains even from things that are lawful. Therefore continence is not a virtue.
On the contrary, Every praiseworthy habit would seem to be a virtue. Now such is continence, for Andronicus says [De Affectibus] that "continence is a habit unconquered by pleasure." Therefore continence is a virtue.
I answer that, The word "continence" is taken by various people in two ways. For some understand continence to denote abstention from all venereal pleasure: thus the Apostle joins continence to chastity (Galatians 5:23). On this sense perfect continence is virginity in the first place, and widowhood in the second. Wherefore the same applies to continence understood thus, as to virginity which we have stated above (Question 152, Article 3) to be a virtue. Others, however, understand continence as signifying that whereby a man resists evil desires, which in him are vehement. On this sense the Philosopher takes continence (Ethic. vii, 7), and thus also it is used in the Conferences of the Fathers (Collat. xii, 10,11). In this way continence has something of the nature of a virtue, in so far, to wit, as the reason stands firm in opposition to the passions, lest it be led astray by them: yet it does not attain to the perfect nature of a moral virtue, by which even the sensitive appetite is subject to reason so that vehement passions contrary to reason do not arise in the sensitive appetite. Hence the Philosopher says (Ethic. iv, 9) that "continence is not a virtue but a mixture," inasmuch as it has something of virtue, and somewhat falls short of virtue.
If, however, we take virtue in a broad sense, for any principle of commendable actions, we may say that continence is a virtue.
Reply to Objection 1. The Philosopher includes continence in the same division with virtue in so far as the former falls short of virtue.
Reply to Objection 2. Properly speaking, man is that which is according to reason. Wherefore from the very fact that a man holds [tenet se] to that which is in accord with reason, he is said to contain himself. Now whatever pertains to perversion of reason is not according to reason. Hence he alone is truly said to be continent who stands to that which is in accord with right reason, and not to that which is in accord with perverse reason. Now evil desires are opposed to right reason, even as good desires are opposed to perverse reason. Wherefore he is properly and truly continent who holds to right reason, by abstaining from evil desires, and not he who holds to perverse reason, by abstaining from good desires: indeed, the latter should rather be said to be obstinate in evil.
Reply to Objection 3. The gloss quoted takes continence in the first sense, as denoting a perfect virtue, which refrains not merely from unlawful goods, but also from certain lawful things that are lesser goods, in order to give its whole attention to the more perfect goods.
Source: Summa Theologica (New Advent)