Rationalism

Rationalism is the epistemological position that reason, not sense-experience, is the primary source and test of knowledge — that the intellect can grasp truths about reality with a certainty the senses can’t provide. In its strong early-modern form it takes mathematics as the model of knowledge: start from self-evident truths and deduce the rest. Historically it’s the continental-European counterpart to British empiricism (knowledge from the senses) — a contrast this spoke will fill in as empiricist sources arrive.

In the founding corpus

• rene-descartes — grounds knowledge in the cogito, an indubitable truth reached by pure reason, and trusts “clear and distinct” ideas over the senses. The dualism follows from this intellect-first picture.
• baruch-spinoza — the strongest instance: the Ethics is written more geometrico (definitions → axioms → propositions → proofs), the monist conviction that reality is fully intelligible to reason, with no brute facts.

Both hold that the structure of reality answers to the structure of thought. Nietzsche later attacks exactly this faith with his perspectivism — the claim that there is no reason’s-eye “view from nowhere” to reach.

Related

rene-descartes · baruch-spinoza · mind-body-problem · western-philosophy · nihilism