Objection 1. It would seem that "a constant man" [Cap. Ad audientiam, De his quae vi.] cannot be compelled by fear. Because the nature of a constant man is not to be agitated in the midst of dangers. Since then fear is "agitation of the mind occasioned by imminent danger," it would seem that he is not compelled by fear.
Objection 2. Further, "Of all fearsome things death is the limit," according to the Philosopher (Ethic. iii, 6), as though it were the most perfect of all things that inspire fear. But the constant man is not compelled by death, since the brave face even mortal dangers. Therefore no fear influences a constant man.
Objection 3. Further, of all dangers a good man fears most that which affects his good name. But the fear of disgrace is not reckoned to influence a constant man, because, according to the law (vii, ff, de eo quod metus, etc.), "fear of disgrace is not included under the ordinance, 'That which is done through fear'" [Dig. iv, 2, Quod metus causa]. Therefore neither does any other kind of fear influence a constant man.
Objection 4. Further, in him who is compelled by fear, fear leaves a sin, for it makes him promise what he is unwilling to fulfill, and thus it makes him lie. But a constant man does not commit a sin, not even a very slight one, for fear. Therefore no fear influences a constant man.
On the contrary, Abraham and Isaac were constant. Yet they were influenced by fear, since on account of fear each said that his wife was his sister (Genesis 12:12; 26:7).
Further, wherever there is mixed violence, it is fear that compels. But however constant a man may be he may suffer violence of that kind, for if he be on the sea, he will throw his merchandise overboard if menaced with shipwreck. Therefore fear can influence a constant man.
I answer that, By fear influencing a man we mean his being compelled by fear. A man is compelled by fear when he does that which otherwise he would not wish to do, in order to avoid that which he fears. Now the constant differs from the inconstant man in two respects. First, in respect of the quality of the danger feared, because the constant man follows right reason, whereby he knows whether to omit this rather than that, and whether to do this rather than that. Now the lesser evil or the greater good is always to be chosen in preference; and therefore the constant man is compelled to bear with the lesser evil through fear of the greater evil, but he is not compelled to bear with the greater evil in order to avoid the lesser. But the inconstant man is compelled to bear with the greater evil through fear of a lesser evil, namely to commit sin through fear of bodily suffering; whereas on the contrary the obstinate man cannot be compelled even to permit or to do a lesser evil, in order to avoid a greater. Hence the constant man is a mean between the inconstant and the obstinate. Secondly, they differ as to their estimate of the threatening evil, for a constant man is not compelled unless for grave and probable reasons, while the inconstant man is compelled by trifling motives: "The wicked man seeth when no man pursueth" (Proverbs 28:1).
Reply to Objection 1. The constant man, like the brave man, is fearless, as the Philosopher states (Ethic. iii, 4), not that he is altogether without fear, but because he fears not what he ought not to fear, or where, or when he ought not to fear.
Reply to Objection 2. Sin is the greatest of evils, and consequently a constant man can nowise be compelled to sin; indeed a man should die rather than suffer the like, as again the Philosopher says (Ethic. iii, 6,9). Yet certain bodily injuries are less grievous than certain others; and chief among them are those which relate to the person, such as death, blows, the stain resulting from rape, and slavery. Wherefore the like compel a constant man to suffer other bodily injuries. They are contained in the verse: "Rape, status, blows, and death." Nor does it matter whether they refer to his own person, or to the person of his wife or children, or the like.
Reply to Objection 3. Although disgrace is a greater injury it is easy to remedy it. Hence fear of disgrace is not reckoned to influence a constant man according to law.
Reply to Objection 4. The constant man is not compelled to lie, because at the time he wishes to give; yet afterwards he wishes to ask for restitution, or at least to appeal to the judge, if he promised not to ask for restitution. But he cannot promise not to appeal, for since this is contrary to the good of justice, he cannot be compelled thereto, namely to act against justice.
Source: Summa Theologica (New Advent)