Objection 1. It seems that fortitude does not deal chiefly with sudden occurrences. For it would seem that things occur suddenly when they are unforeseen. But Tully says (De Invent. Rhet. ii) that "fortitude is the deliberate facing of danger, and bearing of toil." Therefore fortitude does not deal chiefly with sudden happenings.
Objection 2. Further, Ambrose says (De Offic. i): "The brave man is not unmindful of what may be likely to happen; he takes measures beforehand, and looks out as from the conning-tower of his mind, so as to encounter the future by his forethought, lest he should say afterwards: This befell me because I did not think it could possibly happen." But it is not possible to be prepared for the future in the case of sudden occurrences. Therefore the operation of fortitude is not concerned with sudden happenings.
Objection 3. Further, the Philosopher says (Ethic. iii, 8) that the "brave man is of good hope." But hope looks forward to the future, which is inconsistent with sudden occurrences. Therefore the operation of fortitude is not concerned with sudden happenings.
On the contrary, The Philosopher says (Ethic. iii, 8) that "fortitude is chiefly about sudden dangers of death."
I answer that, Two things must be considered in the operation of fortitude. One is in regard to its choice: and thus fortitude is not about sudden occurrences: because the brave man chooses to think beforehand of the dangers that may arise, in order to be able to withstand them, or to bear them more easily: since according to Gregory (Hom. xxv in Evang.), "the blow that is foreseen strikes with less force, and we are able more easily to bear earthly wrongs, if we are forearmed with the shield of foreknowledge." The other thing to be considered in the operation of fortitude regards the display of the virtuous habit: and in this way fortitude is chiefly about sudden occurrences, because according to the Philosopher (Ethic. iii, 8) the habit of fortitude is displayed chiefly in sudden dangers: since a habit works by way of nature. Wherefore if a person without forethought does that which pertains to virtue, when necessity urges on account of some sudden danger, this is a very strong proof that habitual fortitude is firmly seated in his mind.
Yet is it possible for a person even without the habit of fortitude, to prepare his mind against danger by long forethought: in the same way as a brave man prepares himself when necessary. This suffices for the Replies to the Objections.
Source: Summa Theologica (New Advent)