Critique of Pure Reason

Kant's main work, which asks what reason can know independently of experience and sets limits to metaphysics. He argues that we can have synthetic a priori knowledge, as in mathematics and the basic principles of physics, only because the mind supplies the forms of experience. In his 'Copernican' turn, objects conform to our ways of knowing rather than the reverse. The Transcendental Aesthetic treats space and time as forms of our intuition. The Analytic derives twelve categories of the understanding, such as cause and substance, and argues in the Transcendental Deduction that they hold necessarily for any experience we can have. The result is transcendental idealism: we know things as they appear to us, never things in themselves. The Dialectic then shows how reason goes astray when it reaches beyond experience: the paralogisms about the soul, the four antinomies about the world, including freedom against determinism, and the critique of the ontological, cosmological and design proofs of God.

It is the turning point of modern philosophy, and nearly every later school, from German Idealism to analytic philosophy, defines itself by its response to it.