Hume had proved that the law of causality is not analytic, and had inferred that we could not be certain of its truth. Kant accepted the view that it is synthetic, but nevertheless maintained that it is known a priori. He maintained that arithmetic and geometry are synthetic, but are likewise a priori. He was thus led to formulate his problem in these terms: How are synthetic judgements a priori possible? The answer to this question, with its consequences, constitutes the main theme of The Critique of Pure Reason.
