In Book I of Plato's Republic , Cephalus defines ‘justice’ as speaking the truth and paying one's debts. Socrates quickly refutes this account by suggesting that it would be wrong to repay certain debts for example, to return a borrowed weapon to a friend who is not in his right mind. Socrates' point is not that repaying debts is without moral import; rather, he wants to show that it is not always right to repay one's debts, at least not exactly when the one to whom the debt is owed demands repayment. What we have here is a conflict between two moral norms: repaying one's debts and protecting others from harm. And in this case, Socrates maintains that protecting others from harm is the norm that takes priority.
Nearly twenty-four centuries later, Jean-Paul Sartre described a moral conflict the resolution of which was, to many, less obvious than the resolution to the Platonic conflict. Sartre (1957) tells of a student whose brother had been killed in the German offensive of 1940. The student wanted to avenge his brother and to fight forces that he regarded as evil. But the student's mother was living with him, and he was her one consolation in life. The student believed that he had conflicting obligations. Sartre describes him as being torn between two kinds of morality: one of limited scope but certain efficacy, personal devotion to his mother; the other of much wider scope but uncertain efficacy, attempting to contribute to the defeat of an unjust aggressor.