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Reliance on reason {Lat. ratio} as the only reliable source of human knowledge. In the most general application, rationalism offers a naturalistic alternative to appeals to religious accounts of human nature and conduct.

More specifically, rationalism is the epistemological theory that significant knowledge of the world can best be achieved by a priori means; it therefore stands in contrast to empiricism. Prominent rationalists of the modern period include Descartes, Spinoza, and Leibniz.

(Via Philosophical Dictionary)