Prior to Kant, it was thought that all "a priori" knowledge must be analytic. Kant, however, argues that our knowledge of mathematics, of the first principles of natural science, and of metaphysics, is both "a priori" and synthetic. The peculiar nature of this knowledge cries out for explanation. The central problem of the "Critique" is therefore to answer the question: "How are synthetic "a priori" judgements possible?" It is a "matter of life and death" to metaphysics and to human reason, Kant argues, that the grounds of this kind of knowledge be explained.