Upon exactly the same ground is based the duty of keeping one's promises, viz., that our fellow-men build on our promises. Promises made in a legal form are called contracts and can be enforced. Promises not made in legal form are equally binding from a moral point of view. It should be borne in mind, however, that conditional promises are canceled when the stipulated conditions do not occur, and, furthermore, that there are certain tacit conditions implied in all promises whatsoever. A person who has promised to visit a friend on a certain day and dies in the interval is not supposed to have broken his promise; nor if any one makes a similar promise and a heavy snowstorm should block the roads or if he should be confined to his bed by sickness is he likely to be accused of breaking his promise. The physical possibility of fulfilling them is a tacit condition in all promises. It is also a tacit condition in all promises that it shall be morally possible or consistent with morality to keep them. A young man who has promised to join a gang of burglars in an attack on a bank and who repents at the last moment is morally justified in refusing to keep his pledge. His crime consisted in having made the promise in the first place, not in refusing to fulfill it at the last moment. A person, however, who promises to pay usurious interest on a loan of money and who then takes advantage of the laws against usury to escape payment is a double-dyed rogue, for his intention is to cheat, and he uses the cloak of virtue as a screen in order to cheat with impunity. Let the teacher discuss the casuistical question whether it is right to keep a promise made to robbers--e. g., if we should fall into the hands of brigands, and they should make it a condition of our release that we shall not betray their hiding-place.