Thomas Aquinas

“ 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. ”

“ Therefore perseverance is a part of temperance rather than of fortitude.
Objection 2: Further, every part of a moral virtue is about certain passions which that virtue moderates. Now perseverance does not imply moderation of the passions: since the more violent the passions, the more praiseworthy is it to persevere in accordance with reason. Therefore it seems that perseverance is a part not of a moral virtue, but rather of prudence which perfects the reason. ”

“ The virtue of perseverance, so far as it is concerned, inclines one to persevere: yet since it is a habit, and a habit is a thing one uses at will, it does not follow that a person who has the habit of virtue uses it unchangeably until death. ”

“ 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. ”

“ 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. ”

“ Wherefore it follows of necessity that every virtue which has a title to praise for the firm endurance of something difficult must be annexed to fortitude as secondary to principal virtue. Now the endurance of difficulty arising from delay in accomplishing a good work gives perseverance its claim to praise: nor is this so difficult as to endure dangers of death. Therefore perseverance is annexed to fortitude, as secondary to principal virtue. ”

“ Hence by falling into sin, so far as he is concerned man makes himself to be persevering in sin, unless he be delivered by God's grace. On the other hand, by doing good he does not make himself to be persevering in good, because he is able, by himself, to sin: wherefore he needs the help of grace for that end. ”

“ 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. ”

“ 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. ”