Immanuel Kant published the *Kritik der reinen Vernunft* in 1781, at the age of fifty-seven. He never married, almost never left his native Königsberg, and kept university hours of such regularity that neighbours, by legend, set their clocks by his walks. Out of this rigid routine came the 'Copernicus of philosophy'.

Kant himself reports that reading Hume had woken him from his 'dogmatic slumber'. Rationalists had thought reason could on its own reach the structure of reality; empiricists, by insisting that all knowledge derives from experience, had threatened to shrink reason past usefulness. When Hume showed that causality could not be found by reason, the certainty of natural science seemed to dissolve. Kant's question is direct: how is mathematics possible? How is natural science possible?

The answer is the 'Copernican turn'. Assume the opposite of the usual assumption: not that knowledge conforms to objects, but that objects must conform to knowledge. We perceive a world only through space and time (forms of intuition) and through categories such as causality, substance, and unity (concepts of the understanding). These structures do not come from experience — they are *a priori* — yet they make experience possible. Synthetic *a priori* judgements — at once necessary and informative about the world — are possible by virtue of this scaffolding.

The price of the move is a hard limit. Reason can know only the realm of *appearances* — the world as it shows itself to us. The *thing in itself*, the *noumenon*, is unknowable. The traditional metaphysical objects — God, the immortality of the soul, free will — in the merely speculative use of pure reason generate contradictions, the antinomies. Kant thereby places a boundary around the old metaphysical claims, and yet — on quite different grounds — re-opens space for them in the *practical* use of reason.

The book's effect is hard to overstate. Fichte, Schelling, and Hegel begin from it directly; phenomenology, analytic philosophy, and philosophy of science were built in argument with it. 'Before Kant and after Kant' is one of the genuine inflection points of modern philosophy — the moment at which modernity becomes the project of a reason that is conscious of its own limits.