"Critique of pure reason" by Immanuel Kant,1781

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To establish the possibility of metaphysics as a science,it must be shown that synthesis a priori truths are possible.
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Synthetic a priori truths are universally and necessarily true -hence a priori-,but their necessity cannot be derived by analysis of the meanings of such truths -hence,they are synthetic-.
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The two sources of knowledge are sensibility and understanding.
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Space and time are the priori forms of sensibility -intuition- ;
we are so constituted that we cannot perceive anything at all except by casting it into the forms of space and time.
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The a priori conditions of our understanding are called the categories of our understanding : the categories
of quantity are unity,plurality,and totality ;
of quality : reality,negation,and limitation ;
of relation : substance and accident,cause and effect,and reciprocity between agent and patient ;
of modality : possibility-impossibility,existence-unexistence,and necessity-contingency.
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The principles of science which serve as presuppositions are synthetic a priori ;
the possibility of such principles is based upon the use of a priori forms of intuition together with the categories of the understanding.
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