Metaphysics and Epistemology

Kant's most widely read and most influential book is Critique of Pure Reason [1] (1781), which proceeds from a remarkably simple thought experiment. He said, try to imagine something that exists in no time and has no extent in space. The human mind cannot produce such an idea—time and space are fundamental forms of perception that exist as innate structures of the mind. Nothing can be perceived except through these forms, and the limits of physics are the limits of the fundamental structure of the mind. On Kant's view, therefore, there are something like innate ideas—a priori knowledge of some things (space and time)—since the mind must possess these categories in order to be able to understand the buzzing mass of raw, uninterpreted sensory experience which presents itself to our consciousness. Secondly, it removes the actual world (which Kant called the noumenal world, or noumena) from the arena of human perception—since everything we perceive is filtered through the forms of space and time we can never really "know" the real world.
Kant termed his critical philosophy "transcendental idealism While the exact interpretation of this phrase is contentious, one way to start to understand it is through Kant's comparison in the second preface to the "Critique of Pure Reason" of his critical philosophy to Copernicus' revolution in astronomy. Kant writes: "Hitherto it has been assumed that all our knowledge must conform to objects. But all attempts to extend our knowledge of objects by establishing something in regard to them a priori, by means of concepts, have, on this assumption, ended in failure. We must therefore make trial whether we may not have more success in the tasks of metaphysics, if we suppose that objects must conform to our knowledge" [Bxvi]. Just as Copernicus revolutionized astronomy by changing the point of view, Kant's critical philosophy asks what the a priori conditions for our knowledge of objects in the world might be. Transcendental idealism describes this method of seeking the conditions of the possibility of our knowledge of the world.
Kant's "transcendental idealism" should be distinguished from idealistic systems such as Berkeley's. While Kant claimed that phenomena depend upon the conditions of sensibility, space and time, this thesis is not equivalent to mind-dependence in the sense of Berkeley's idealism. For Berkeley, something is an object only if it can be perceived. For Kant, on the other hand, perception does not provide the criterion for the existence of objects. Rather, the conditions of sensibility - space and time - provide the "epistemic conditions", to borrow a phrase from Henry Allison, required for us to know objects in the phenomenal world.
Kant had wanted to discuss metaphysical systems but discovered "the scandal of philosophy"—you cannot decide what the proper terms for a metaphysical system are until you have defined the field, and you cannot define the field until you have defined the limit of the field of physics first. 'Physics' in this sense means, roughly, the discussion of the perceptible world.