Objection 1. It seems that fortitude is not a cardinal virtue. For, as stated above (Question 123, Article 10), anger is closely allied with fortitude. Now anger is not accounted a principal passion; nor is daring which belongs to fortitude. Therefore neither should fortitude be reckoned a cardinal virtue.
Objection 2. Further, the object of virtue is good. But the direct object of fortitude is not good, but evil, for it is endurance of evil and toil, as Tully says (De Inv. Rhet. ii). Therefore fortitude is not a cardinal virtue.
Objection 3. Further, the cardinal virtues are about those things upon which human life is chiefly occupied, just as a door turns upon a hinge (cardine). But fortitude is about dangers of death which are of rare occurrence in human life. Therefore fortitude should not be reckoned a cardinal or principal virtue.
On the contrary, Gregory (Moral. xxii), Ambrose in his commentary on Luke 6:20, and Augustine (De Moribus Eccl. xv), number fortitude among the four cardinal or principal virtues.
I answer that, As stated above (Prima Secundae Partis, Question 61, Article 3 and Article 4), those virtues are said to be cardinal or principal which have a foremost claim to that which belongs to the virtues in common. And among other conditions of virtue in general is one that is stated to "act steadfastly", according to Ethic. ii, 4. Now fortitude above all lays claim to praise for steadfastness. Because he that stands firm is so much the more praised, as he is more strongly impelled to fall or recede. Now man is impelled to reced from that which is in accordance with reason, both by the pleasing good and the displeasing evil. But bodily pain impels him more strongly than pleasure. For Augustine says (QQ. 83, qu. 36): "There is none that does not shun pain more than he desires pleasure. For we perceive that even the most untamed beasts are deterred from the greatest pleasures by the fear of pain." And among the pains of the mind and dangers those are mostly feared which lead to death, and it is against them that the brave man stands firm. Therefore fortitude is a cardinal virtue.
Reply to Objection 1. Daring and anger do not cooperate with fortitude in its act of endurance, wherein its steadfastness is chiefly commended: for it is by that act that the brave man curbs fear, which is a principal passion, as stated above (Prima Secundae Pars, Question 25, Article 4).
Reply to Objection 2. Virtue is directed to the good of reason which it behooves to safeguard against the onslaught of evils. And fortitude is directed to evils of the body, as contraries which it withstands, and to the good of reason, as the end, which it intends to safeguard.
Reply to Objection 3. Though dangers of death are of rare occurrence, yet the occasions of those dangers occur frequently, since on account of justice which he pursues, and also on account of other good deeds, man encounters mortal adversaries.
Source: Summa Theologica (New Advent)