rationalis

Rationalis in Latin means 'of or belonging to reason,' from ratio (reckoning, account, reason). Ratio itself comes from reri (to reckon, to think). In ancient Rome, rationalis was an accounting term before it was a philosophical one — a rationalis was a financial officer. The word meant calculation first and cognition second.

Rene Descartes is the father of modern rationalism. His Meditations on First Philosophy (1641) argued that the senses deceive — you might be dreaming, a demon might be tricking you — but reason can reach certainty. His method of systematic doubt led to the cogito: I think, therefore I am. From that foundation, he attempted to deduce the existence of God and the external world through pure reason alone. Baruch Spinoza and Gottfried Wilhelm Leibniz followed, each building elaborate metaphysical systems from rational principles.

The rationalist-empiricist debate defined philosophy for a century. Rationalists said some knowledge is innate — mathematical truths, logical axioms, the idea of God. Empiricists said all knowledge comes from experience. Immanuel Kant, in the Critique of Pure Reason (1781), attempted a synthesis: the mind has innate structures (space, time, causality) that organize experience. Kant did not end the debate. He complicated it permanently.

In everyday English, 'rational' and 'rationalist' have drifted far from Descartes. Rational now means reasonable, logical, not emotional. A rational decision. A rational person. The philosophical meaning — that reason alone, without sensory experience, can deliver knowledge about reality — survives only in philosophy departments. The word kept the reputation and lost the radicalism.