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.

Where the faith is stated outright (added 2026-09-05). spinoza-ethics Part I puts it in an axiom — "A true idea must correspond with its ideate or object" (Ax. VI) — asserted, never argued for, with 36 propositions standing on it. That is the premise of this whole position written down as something too obvious to defend, which is exactly the place Nietzsche's attack lands.

A narrower line than the one drawn above (added 2026-10-06)

The opening of this page sets reason against the senses. sep-rationalism-empiricism (T1) draws the line elsewhere. Both camps accept intuition and deduction, so that thesis "does not divide" them; what makes someone a rationalist is accepting innate knowledge or innate concepts in some subject area. The entry also relativises both positions to a subject, so a person can be a rationalist about mathematics and an empiricist about physics. The older framing above is kept because it is the usual textbook one; the SEP's version is the more exact one, and the spoke synthesis records the gap.

Related

empiricism · sep-rationalism-empiricism · rene-descartes · baruch-spinoza · spinoza-ethics · mind-body-problem · western-philosophy · nihilism