Fortitude, fortitudo, is firmness of mind in the face of danger and difficulty, and Aquinas holds, following Aristotle, that its principal act is endurance rather than attack, since standing firm against an evil one cannot remove is harder than striking at one that can be overcome. Its supreme instance in the Christian tradition is martyrdom. It is the virtue of the mean between cowardice and rashness, and the tradition is careful that fearlessness is not itself a virtue: the courageous person fears what should be feared and acts rightly nonetheless. Its subordinate parts are given as magnanimity, magnificence, patience and perseverance. In the four-cardinal scheme fortitude and temperance govern the passions, the irascible and the concupiscible respectively, while prudence governs the reason and justice the will.
The philosopher Alasdair MacIntyre, in After Virtue, has discussed this shift from Aristotle to Aquinas as characteristic of a broader Christian transformation of the classical virtues: where Aristotle's model of courage in the Nicomachean Ethics centers on the soldier who attacks danger on the battlefield, Aquinas relocates fortitude's highest expression to the martyr's endurance of suffering inflicted by another, a passive rather than active heroism that would have struck Aristotle as a lesser, more marginal case of the virtue rather than its supreme instance, illustrating how thoroughly the Christian tradition reordered an inherited philosophical scheme around its own distinctive convictions.