I answer that, Chastity takes its name from the fact that reason "chastises" concupiscence, which, like a child, needs curbing, as the Philosopher states (Ethic. iii, 12). Now the essence of human virtue consists in being something moderated by reason, as shown above (I-II, Q. 64, A. 1). Therefore it is evident that chastity is a virtue.
Reply Obj. 1: Chastity does indeed reside in the soul as its subject, though its matter is in the body. For it belongs to chastity that a man make moderate use of bodily members in accordance with the judgment of his reason and the choice of his will.
Reply Obj. 2: As Augustine says (De Civ. Dei i, 18), "so long as her mind holds to its purpose, whereby she has merited to be holy even in body, not even the violence of another's lust can deprive her body of its holiness, which is safeguarded by her persevering continency." He also says (De Civ. Dei i, 18) that "in the mind there is a virtue which is the companion of fortitude, whereby it is resolved to suffer any evil whatsoever rather than consent to evil."
Reply Obj. 3: As Augustine says (Contra Julian. iv, 3), "it is impossible to have any true virtue unless one be truly just; nor is it possible to be just unless one live by faith." Whence he argues that in unbelievers there is neither true chastity, nor any other virtue, because, to wit, they are not referred to the due end, and as he adds (Contra Julian. iv, 3) "virtues are distinguished from vices not by their functions," i.e. their acts, "but by their ends."
Reply Obj. 4: Chastity is a virtue in so far as it works in accordance with reason, but in so far as it delights in its act, it is reckoned among the fruits. _______________________
SECOND ARTICLE [II-II, Q. 151, Art. 2]
Whether Chastity Is a General Virtue?
Objection 1: It would seem that chastity is a general virtue. For Augustine says (De Mendacio xx) that "chastity of the mind is the well-ordered movement of the mind that does not prefer the lesser to the greater things." But this belongs to every virtue. Therefore chastity is a general virtue.
Obj. 2: Further, "Chastity" takes its name from "chastisement" [*Cf. A. 1]. Now every movement of the appetitive part should be chastised by reason. Since, then, every moral virtue curbs some movement of the appetite, it seems that every moral virtue is chastity.
Obj. 3: Further, chastity is opposed to fornication. But fornication seems to belong to every kind of sin: for it is written (Ps. 72:27): "Thou shalt destroy [Vulg.: 'hast destroyed'] all them that go awhoring from [Douay: 'are disloyal to'] Thee." Therefore chastity is a general virtue.