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 a peace in 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.