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Four kinds of knowing, one crucial combination
💡 Kant splits knowledge along two axes — a priori/a posteriori and analytic/synthetic — to isolate the one prized combination: synthetic a priori.
Kant's whole argument turns on two distinctions most people blur together. The first is about source: knowledge is a priori if it's justified independently of experience (like '2+2=4'), and a posteriori if it depends on observation ('the sky is cloudy today').
The second is about content: a judgment is analytic if the predicate is already hidden inside the subject ('all bodies are extended' — being extended is part of what 'body' means), and synthetic if it joins two genuinely distinct concepts to give new information ('all bodies have weight').
Cross these two and you get four boxes. Three are easy — analytic a priori (definitions), synthetic a posteriori (ordinary facts), and the impossible analytic a posteriori. The prize box is synthetic a priori: informative and necessary. Kant insisted this box isn't empty — it holds mathematics, geometry, and principles like 'every event has a cause.' The rest of the book is his attempt to explain how the mind fills it.
Source: Critique of Pure Reason — Immanuel Kant