Immanuel Kant

Immanuel Kant (1724-1804) never travelled more than 100 kilometres from his birthplace of Konigsberg, yet his Critique of Pure Reason (1781) produced what he called a "Copernican revolution" in philosophy: our minds do not passively receive reality, they actively construct it through innate categories of space, time, and causation. We can never know the "thing-in-itself," only how it appears to us. His Groundwork of the Metaphysics of Morals introduced the Categorical Imperative — "Act only according to that maxim whereby you can at the same time will that it should become a universal law" — the most rigorous secular foundation for ethics ever proposed. He unified empiricism and rationalism into a synthesis that shaped every subsequent philosophical movement: Hegel, Marx, phenomenology, and modern analytic philosophy alike.