From Fundamental Principles of the Metaphysics of Morals. “Kant had discovered an entirely new human faculty, namely judgment,” wrote Hannah Arendt about this text, in which the Prussian philosopher presents his concept of the categorical imperative, which dictates that one should always respect the humanity in others and act only in accordance with rules that apply to everyone. “But at the same time,” Arendt wrote, “the question of right and wrong is to be decided by neither taste nor judgment but by reason alone.”
Everyone must admit that if a law is to have moral force, i.e., to be the basis of an obligation, it must carry with it absolute necessity; that, for example, the precept “Thou shalt not lie” is not valid for men alone, as if other rational beings had no need to observe it; and so with all the other moral laws properly so called; that, therefore, the basis of obligation must not be sought in the nature of man, or in the circumstances in the world in which he is placed, but a priori simply in the conceptions of pure reason; and although any other precept that is founded on principles of mere experience may be in certain respects universal, in as far as it rests even in the least degree on an empirical basis, perhaps only as to a motive, such a precept, while it may be a practical rule, can never be called a moral law.
Thus not only are moral laws with their principles essentially distinguished from every other kind of practical knowledge in which there is anything empirical, but all moral philosophy rests wholly on its pure part. No doubt these laws require a judgment sharpened by experience, in order on the one hand to distinguish in what cases they are applicable and on the other to procure for them access to the will of the man.
In order that an action should be morally good, it is not enough that it conform to the moral law, but it must also be done for the sake of the law, otherwise the conformity to law is only very contingent and uncertain; since a principle that is not moral, although it may now and then produce actions conformable to the law, will also often produce actions that contradict it.