Even when rules of conduct have been organised into a system and become a definite moral code, it may be impossible to apply them in concrete moral situations. If we attempt to apply the Ten Commandments, we soon find that it may be impossible to observe one commandment without breaking another. Conflicts may take place between two commandments. In all such cases of conflict, we are bound to ask, Which law has superior authority? Which is the greatest commandment? Which is the supreme principle of duty?
A comprehensive principle of duty was suggested by Kant and formulated thus: "Act only on that maxim which thou canst at the same time will to become a universal law." Unless I am ready to will that the act which I am going to perform should become a universal law, and be performed by all other human beings, my act is not right. Telling the truth is right, because I can will that everybody should always tell the truth. Committing suicide is wrong, because if I willed that suicide should become universal, the whole human race would disappear. And I cannot will that. Giving way to utter idleness is wrong, for I cannot will that all men should live in idleness and fail to develop their faculties.
Kant illustrates the working of the principle in detail. "A man finds himself forced by necessity to borrow money. He knows that he will not be able to repay it, but sees also that nothing will be lent to him, unless he promises stoutly to repay it in a definite time. He desires to make this promise, but he has still so much conscience as to ask himself: Is it not unlawful and inconsistent with duty to get
