The question that launched modern philosophy

In 1781 Immanuel Kant set out to referee a centuries-old fight. Rationalists claimed reason alone could reveal reality's deepest truths — God, the soul, the cosmos. Empiricists like Hume replied that all knowledge comes from experience, which meant such grand claims were unprovable. Kant thought both were half right, and that philosophy needed a critique — an examination of reason by reason itself before trusting any of its conclusions.

His guiding puzzle: how are synthetic a priori judgments possible? A judgment is a priori if we know it independently of experience, and synthetic if it genuinely adds new information rather than just unpacking a definition. Kant believed mathematics and the foundations of physics are full of such judgments — necessary truths that still tell us something real about the world.

Explaining how that's even possible became the whole project. If we can pin down the conditions that make synthetic a priori knowledge work, Kant argued, we simultaneously discover the limits of what human reason can legitimately know — and where it starts talking nonsense.