Objection 1. It seems that perseverance is not a virtue. For, according to the Philosopher (Ethic. vii, 7), continency is greater than perseverance. But continency is not a virtue, as stated in Ethic. iv, 9. Therefore perseverance is not a virtue.
Objection 2. Further, "by virtue man lives aright," according to Augustine (De Lib. Arb. ii, 19). Now according to the same authority (De Persever. i), no one can be said to have perseverance while living, unless he persevere until death. Therefore perseverance is not a virtue.
Objection 3. Further, it is requisite of every virtue that one should persist unchangeably in the work of that virtue, as stated in Ethic. ii, 4. But this is what we understand by perseverance: for Tully says (De Invent. Rhet. ii) that "perseverance is the fixed and continued persistence in a well-considered purpose." Therefore perseverance is not a special virtue, but a condition of every virtue.
On the contrary, Andronicus [Chrysippus: in De Affect.] says that "perseverance is a habit regarding things to which we ought to stand, and those to which we ought not to stand, as well as those that are indifferent." Now a habit that directs us to do something well, or to omit something, is a virtue. Therefore perseverance is a virtue.
I answer that, According to the Philosopher (Ethic. ii, 3), "virtue is about the difficult and the good"; and so where there is a special kind of difficulty or goodness, there is a special virtue. Now a virtuous deed may involve goodness or difficulty on two counts. First, from the act's very species, which is considered in respect of the proper object of that act: secondly, from the length of time, since to persist long in something difficult involves a special difficulty. Hence to persist long in something good until it is accomplished belongs to a special virtue.
Accordingly just as temperance and fortitude are special virtues, for the reason that the one moderates pleasures of touch (which is of itself a difficult thing), while the other moderates fear and daring in connection with dangers of death (which also is something difficult in itself), so perseverance is a special virtue, since it consists in enduring delays in the above or other virtuous deeds, so far as necessity requires.
Reply to Objection 1. The Philosopher is taking perseverance there, as it is found in one who bears those things which are most difficult to endure long. Now it is difficult to endure, not good, but evil. And evils that involve danger of death, for the most part are not endured for a long time, because often they soon pass away: wherefore it is not on this account that perseverance has its chief title to praise. Among other evils foremost are those which are opposed to pleasures of touch, because evils of this kind affect the necessaries of life: such are the lack of food and the like, which at times call for long endurance. Now it is not difficult to endure these things for a long time for one who grieves not much at them, nor delights much in the contrary goods; as in the case of the temperate man, in whom these passions are not violent. But they are most difficult to bear for one who is strongly affected by such things, through lacking the perfect virtue that moderates these passions. Wherefore if perseverance be taken in this sense it is not a perfect virtue, but something imperfect in the genus of virtue. On the other hand, if we take perseverance as denoting long persistence in any kind of difficult good, it is consistent in one who has even perfect virtue: for even if it is less difficult for him to persist, yet he persists in the more perfect good. Wherefore such like perseverance may be a virtue, because virtue derives perfection from the aspect of good rather than from the aspect of difficulty.
Reply to Objection 2. Sometimes a virtue and its act go by the same name: thus Augustine says (Tract. in Joan. lxxix): "Faith is to believe without seeing." Yet it is possible to have a habit of virtue without performing the act: thus a poor man has the habit of magnificence without exercising the act. Sometimes, however, a person who has the habit, begins to perform the act, yet does not accomplish it, for instance a builder begins to build a house, but does not complete it. Accordingly we must reply that the term "perseverance" is sometimes used to denote the habit whereby one chooses to persevere, sometimes for the act of persevering: and sometimes one who has the habit of perseverance chooses to persevere and begins to carry out his choice by persisting for a time, yet completes not the act, through not persisting to the end. Now the end is twofold: one is the end of the work, the other is the end of human life. Properly speaking it belongs to perseverance to persevere to the end of the virtuous work, for instance that a soldier persevere to the end of the fight, and the magnificent man until his work be accomplished. There are, however, some virtues whose acts must endure throughout the whole of life, such as faith, hope, and charity, since they regard the last end of the entire life of man. Wherefore as regards these which are the principal virtues, the act of perseverance is not accomplished until the end of life. It is in this sense that Augustine speaks of perseverance as denoting the consummate act of perseverance.
Reply to Objection 3. Unchangeable persistence may belong to a virtue in two ways. First, on account of the intended end that is proper to that virtue; and thus to persist in good for a long time until the end, belongs to a special virtue called perseverance, which intends this as its special end. Secondly, by reason of the relation of the habit to its subject: and thus unchangeable persistence is consequent upon every virtue, inasmuch as virtue is a "quality difficult to change."
Source: Summa Theologica (New Advent)