Kant's philosophy has been called a synthesis of rationalism and empiricism. From rationalism he takes the idea that we can have a priori knowledge of significant truths, but rejects the idea that we can have a priori metaphysical knowledge about the nature of things in themselves, God, or the soul. From empiricism he takes the idea that knowledge is essentially knowledge of experience, but rejects the idea that we cannot learn any necessary truths about experience, and in doing so he rejects Hume's skepticism.

Kant is able to create this synthesis largely thanks to a radical reconception of the nature of knowledge that comes from from experience. Though empiricists and rationalists may have disagreed about the value or certainty of knowledge from experience, they both generally thought of the mind as a neutral receptor: knowledge from experience was simply the report of the senses. Kant points out that our knowledge of experience extends far beyond what the senses can report. Our senses can report sensations, but they cannot give these sensations a structure in space and time, or organize them according to cause and effect. According to Kant, our faculties or sensibility and understanding are largely responsible for what we think of as "knowledge from experience."

By giving our mind this complex internal structure, Kant makes room for a great deal of a priori knowledge. Though the sensations that are the basis for all our experience come from things in themselves, any regularity or structure we find in these sensations comes from our mental concepts. Thus, while Kant does not slip into the idealist position of saying that reality is all a matter of perception, he does claim that the laws of nature are the laws of our mental faculties. For something to be an objective law, it must be synthetic and it must be a priori, and Kant identifies the possibility of synthetic a priori knowledge within the structure of our mental faculties.

If our sense of order and regularity is not something we find in experience, but something we impose upon experience, the study of this order and regularity is a study of our own faculties rather than a study of experience. Kant reconceives the purpose of metaphysics as being one of critique: we must seek to understand how knowledge is structured, and consequently how the various concepts of our mental faculties are organized. This is an important step for philosophy: after Kant published his work, there was been less interest in making extravagant claims about the nature of the universe, and greater emphasis on determining what we can know and on what grounds we can claim to know it.

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