Article — Prümmer, Manuale Theologiae Moralis
Article I. On the Virtue of Abstinence and on Fasting
649. Notion. Abstinence taken generally signifies the withdrawal of food. In this general sense it is not a virtue, but something indifferent, which from circumstances can become good or evil. Thus he who abstains from foods from vanity commits an evil act; but he who abstains from foods on account of the right dictate of reason and virtue performs a good act.1195 The moderate use of foods is prescribed not only by the virtue of abstinence, but also by the medical art, but in different ways. For medicine demands this moderate use principally in order to bodily health, whereas the virtue of abstinence commands the same moderation in order to man's moral good.
It is to be observed that, although abstinence is said to concern the moderate use of foods, it also moderates the use of drink, insofar as drink serves for nourishment; but not the use of intoxicating drink, the moderation of which pertains to the virtue of sobriety. Hence, if someone drinks milk immoderately, he does not sin against sobriety but against abstinence; but if he drinks wine immoderately, he sins against sobriety, not against abstinence.
Definition. Abstinence is a moral virtue inclining one to the moderate use of foods, according as right reason (or faith) shall have determined for the moral good.
If natural right reason is applied as the sole rule of abstinence, there is the natural acquired virtue of abstinence; but if reason illuminated by the virtue of faith is the directing rule, there is the supernatural virtue of abstinence. That abstinence understood in this manner is a true and special virtue is evident, since it rightly orders a special object, namely, the pleasures of foods, which not rarely withdraw man from the good of reason.1196
The act of the virtue of abstinence is fasting and abstinence strictly so called, namely, abstinence from certain determined foods. For the sake of clarity we distribute all things which must be said concerning the virtue of abstinence into four sections: 1. on fasting in general; 2. on the extent of the ecclesiastical law of fasting; 3. on the ecclesiastical law of abstinence; 4. on causes excusing from the law of fasting and of abstinence from meat.
1195 Cf. S. theol. 2, 2, q. 146, a. 1.
1196 Ib., a. 2.