It belongs to the virtue of fortitude to remove any obstacle that withdraws the will from following the reason. Now to be withdrawn from something difficult belongs to the notion of fear, which denotes withdrawal from an evil that entails difficulty… Hence fortitude is chiefly about fear of difficult things, which can withdraw the will from following the reason. And it behooves one not only firmly to bear the assault of these difficulties by restraining fear, but also moderately to withstand them, when, to wit, it is necessary to dispel them altogether in order to free oneself therefrom for the future, which seems to come under the notion of daring. Therefore fortitude is about fear and daring, as curbing fear and moderating daring.
It belongs to the virtue of fortitude to guard the will against being withdrawn from the good of reason through fear of bodily evil. Now it behooves one to hold firmly the good of reason against every evil whatsoever, since no bodily good is equivalent to the good of the reason. Hence fortitude of soul must be that which binds the will firmly to the good of reason in face of the greatest evils: because he that stands firm against great things, will in consequence stand firm against less things, but not conversely. Moreover it belongs to the notion of virtue that it should regard something extreme: and the most fearful of all bodily evils is death, since it does away all bodily goods. Wherefore Augustine says that “the soul is shaken by its fellow body, with fear of toil and pain, lest the body be stricken and harassed with fear of death lest it be done away and destroyed.” Therefore the virtue of fortitude is about the fear of dangers of death.
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 recede 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.