Objection 1. It seems that fortitude is not a virtue. For the Apostle says (2 Corinthians 12:9): "Virtue is perfected in infirmity." But fortitude is contrary to infirmity. Therefore fortitude is not a virtue.
Objection 2. Further, if it is a virtue, it is either theological, intellectual, or moral. Now fortitude is not contained among the theological virtues, nor among the intellectual virtues, as may be gathered from what we have said above (I-II, 57, 2; I-II, 62, 3). Neither, apparently, is it contained among the moral virtues, since according to the Philosopher (Ethic. iii, 7,8): "Some seem to be brave through ignorance; or through experience, as soldiers," both of which cases seem to pertain to act rather than to moral virtue, "and some are called brave on account of certain passions"; for instance, on account of fear of threats, or of dishonor, or again on account of sorrow, anger, or hope. But moral virtue does not act from passion but from choice, as stated above (I-II, 55, 4). Therefore fortitude is not a virtue.
Objection 3. Further, human virtue resides chiefly in the soul, since it is a "good quality of the mind," as stated above (Ethic. iii, 7,8). But fortitude, seemingly, resides in the body, or at least results from the temperament of the body. Therefore it seems that fortitude is not a virtue.
On the contrary, Augustine (De Morib. Eccl. xv, xxi, xxii) numbers fortitude among the virtues.
I answer that, According to the Philosopher (Ethic. ii, 6) "virtue is that which makes its possessor good, and renders his work good." Hence human virtue, of which we are speaking now, is that which makes a man good, and tenders his work good. Now man's good is to be in accordance with reason, according to Dionysius (Div. Nom. iv, 22). Wherefore it belongs to human virtue to make man good, to make his work accord with reason. This happens in three ways: first, by rectifying reason itself, and this is done by the intellectual virtues; secondly, by establishing the rectitude of reason in human affairs, and this belongs to justice; thirdly, by removing the obstacles to the establishment of this rectitude in human affairs. Now the human will is hindered in two ways from following the rectitude of reason. First, through being drawn by some object of pleasure to something other than what the rectitude of reason requires; and this obstacle is removed by the virtue of temperance. Secondly, through the will being disinclined to follow that which is in accordance with reason, on account of some difficulty that presents itself. On order to remove this obstacle fortitude of the mind is requisite, whereby to resist the aforesaid difficulty even as a man, by fortitude of body, overcomes and removes bodily obstacles.
Hence it is evident that fortitude is a virtue, in so far as it conforms man to reason.
Reply to Objection 1. The virtue of the soul is perfected, not in the infirmity of the soul, but in the infirmity of the body, of which the Apostle was speaking. Now it belongs to fortitude of the mind to bear bravely with infirmities of the flesh, and this belongs to the virtue of patience or fortitude, as also to acknowledge one's own infirmity, and this belongs to the perfection that is called humility.
Reply to Objection 2. Sometimes a person performs the exterior act of a virtue without having the virtue, and from some other cause than virtue. Hence the Philosopher (Ethic. iii, 8) mentions five ways in which people are said to be brave by way of resemblance, through performing acts of fortitude without having the virtue. This may be done in three ways. First, because they tend to that which is difficult as though it were not difficult: and this again happens in three ways, for sometimes this is owing to ignorance, through not perceiving the greatness of the danger; sometimes it is owing to the fact that one is hopeful of overcoming dangers--when, for instance, one has often experienced escape from danger; and sometimes this is owing to a certain science and art, as in the case of soldiers who, through skill and practice in the use of arms, think little of the dangers of battle, as they reckon themselves capable of defending themselves against them; thus Vegetius says (De Re Milit. i), "No man fears to do what he is confident of having learned to do well." Secondly, a man performs an act of fortitude without having the virtue, through the impulse of a passion, whether of sorrow that he wishes to cast off, or again of anger. Thirdly, through choice, not indeed of a due end, but of some temporal advantage to be obtained, such as honor, pleasure, or gain, or of some disadvantage to be avoided, such as blame, pain, or loss.
Reply to Objection 3. The fortitude of the soul which is reckoned a virtue, as explained in the Reply to the First Objection, is so called from its likeness to fortitude of the body. Nor is it inconsistent with the notion of virtue, that a man should have a natural inclination to virtue by reason of his natural temperament, as stated above (I-II, 63, 1).
Source: Summa Theologica (New Advent)