Mill on Kant

This remarkable man, whose system of thought will long remain one of the landmarks in the history of philosophical speculation, does, in the treatise in question, lay down a universal first principle as the origin and ground of moral obligation; it is this: "So act, that the rule on which thou actest would admit of being adopted as a law by all rational beings." But when he begins to deduce from this precept any of the actual duties of morality, he fails, almost grotesquely, to show that there would be any contradiction, any logical (not to say physical) impossibility, in the adoption by all rational beings of the most outrageously immoral rules of conduct. All he shows is that the consequences of their universal adoption would be such as no one would choose to incur.
J.S. Mill, Utilitarianism, Chapter 1.