Immanuel Kant

Kant never left the small Prussian city of Königsberg. He took the same walk every afternoon — his neighbors set their clocks by it. From this absolutely ordinary life came the most ambitious philosophical system since Aristotle.

Kant's achievement was to broker the long war between rationalists and empiricists. The rationalists were right that there is necessary, universal knowledge; the empiricists were right that it must somehow involve experience. Kant's answer: the mind imposes its own structure — space, time, causation, substance — on experience. We don't just receive the world; we constitute it. We can never know things-in-themselves, only things as they appear to a structured mind.

His ethics is just as radical. Morality is not about consequences or feelings; it is about acting from duty according to principles you could universalize — the categorical imperative. Treat every person as an end in themselves, never merely as a means.

Almost every philosopher after Kant is either his heir or his opponent. Often both.