Kant's ethical theory as a whole may be best characterised as a fusion of two wholly distinct doctrines. On the one hand his work contains an elaborate investigation into the nature and relations of the concept ‘ought’ – an investigation covering the ground of what may be called ‘Metaphysics of Ethics’. And on the other hand, it contains a peculiar and exclusive doctrine of Practical Ethics. Kant himself seems never to have perceived how distinct these two parts of his work were, and how completely the former is independent of the latter.

To the metaphysical department belongs Kant's recognition of the independence of moral law from natural law. What ought to be is something which perhaps never has been and never will be; and never, by considering what has been or what is, will you discover it. In his own language: the moral law is something a priori and no induction from experience. This doctrine at once distinguishes his system from any so-called naturalistic ethics; from the ethics based on psychological hedonism, or from modern evolutionistic systems.

And to the same department belongs his recognition that ethics must be based on law. Any ethical principle must be universal in the sense that if it is valid for you now, it must be valid for [everyone] else under the same circumstances.