Kantianism is the philosophy of Immanuel Kant.

In epistemology, Kantianism rests upon a sharp distinction between what is knowable, but only provisionally real, and what is unknowable, but really real. These are called the phenomenon and the noumenon, respectively. The phenomenal world consists of objects of the senses, modified by various categories of the intellect. The phenomenal world obeys laws, and these laws (synthetic propositions known a priori), legislate to anything that we can experience precisely because they legislate for anything that WE can experience.

In metaphysics, then, Kantianism involves a profound skepticism about the really real world, inclusive of questions of the existence of a God, or of human free will, or of immortality. But one line of post-Kantian thought suggests that we should overcome our skepticism by believing what is in the line of our needs in these areas. In this way, Kantianism recalls Pascal's wager and prefigures American pragmatism.

In ethics, Kant said that one ought to act in such a way that the principle of one's action could serve as a universal law. He believed that this formulation of the categorical imperative could be shown to be equivalent to another: always treat another person as an end in himself, never solely as a means to an end.

Kant's moral theory is sometimes held up as a paradigm of deontology, and contrasted with the consequentialism of, for example, Utilitarianism.

See also: Neo-Kantianism