Objection 1. It seems that abstinence is not a virtue. For the Apostle says (1 Corinthians 4:20): "The kingdom of God is not in speech but in power [virtute]." Now the kingdom of God does not consist in abstinence, for the Apostle says (Romans 14:17): "The kingdom of God is not meat and drink," where a gloss [Cf. St. Augustine, QQ. Evang. ii, qu. 11 observes that "justice consists neither in abstaining nor in eating." Therefore abstinence is not a virtue.
Objection 2. Further, Augustine says (Confess. x, 11) addressing himself to God: "This hast Thou taught me, that I should set myself to take food as physic." Now it belongs not to virtue, but to the medical art to regulate medicine. Therefore, in like manner, to regulate one's food, which belongs to abstinence, is an act not of virtue but of art.
Objection 3. Further, every virtue "observes the mean," as stated in Ethic. ii, 6,7. But abstinence seemingly inclines not to the mean but to deficiency, since it denotes retrenchment. Therefore abstinence is not a virtue.
Objection 4. Further, no virtue excludes another virtue. But abstinence excludes patience: for Gregory says (Pastor. iii, 19) that "impatience not unfrequently dislodges the abstainer's mind from its peaceful seclusion." Likewise he says (Pastor. iii, 19) that "sometimes the sin of pride pierces the thoughts of the abstainer," so that abstinence excludes humility. Therefore abstinence is not a virtue.
On the contrary, It is written (2 Peter 1:5-6): "Join with your faith virtue, and with virtue knowledge, and with knowledge abstinence"; where abstinence is numbered among other virtues. Therefore abstinence is a virtue.
I answer that, Abstinence by its very name denotes retrenchment of food. Hence the term abstinence may be taken in two ways. First, as denoting retrenchment of food absolutely, and in this way it signifies neither a virtue nor a virtuous act, but something indifferent. Secondly, it may be taken as regulated by reason, and then it signifies either a virtuous habit or a virtuous act. This is the meaning of Peter's words quoted above, where he says that we ought "to join abstinence with knowledge," namely that in abstaining from food a man should act with due regard for those among whom he lives, for his own person, and for the requirements of health.
Reply to Objection 1. The use of and abstinence from food, considered in themselves, do not pertain to the kingdom of God, since the Apostle says (1 Corinthians 8:8): "Meat doth not commend us to God. For neither, if we eat not [Vulgate: 'Neither if we eat... nor if we eat not'], shall we have the less, nor if we eat, shall we have the more," i.e. spiritually. Nevertheless they both belong to the kingdom of God, in so far as they are done reasonably through faith and love of God.
Reply to Objection 2. The regulation of food, in the point of quantity and quality, belongs to the art of medicine as regards the health of the body: but in the point of internal affections with regard to the good of reason, it belongs to abstinence. Hence Augustine says (QQ. Evang. ii, qu. 11): "It makes no difference whatever to virtue what or how much food a man takes, so long as he does it with due regard for the people among whom he lives, for his own person, and for the requirements of his health: but it matters how readily and uncomplainingly he does without food when bound by duty or necessity to abstain."
Reply to Objection 3. It belongs to temperance to bridle the pleasures which are too alluring to the soul, just as it belongs to fortitude to strengthen the soul against fears that deter it from the good of reason. Wherefore, just as fortitude is commended on account of a certain excess, from which all the parts of fortitude take their name, so temperance is commended for a kind of deficiency, from which all its parts are denominated. Hence abstinence, since it is a part of temperance, is named from deficiency, and yet it observes the mean, in so far as it is in accord with right reason.
Reply to Objection 4. Those vices result from abstinence in so far as it is not in accord with right reason. For right reason makes one abstain as one ought, i.e. with gladness of heart, and for the due end, i.e. for God's glory and not one's own.
Source: Summa Theologica (New Advent)