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Descartes approach to reasoning, a commitment to reasoning from first principles, came to be called rationalism. (Malik, 2000, pp 39)

Rationalism is a doctrine that reason alone is the source of knowledge and is independent of experience. Rationalists accept reason as the supreme authority in matters of opinion, belief, or conduct. Kant's dualism, of two causalities, views humans as independent agents having rational choice, while nature ruled by formative causality. See causality. Rationalists stand in contrast to empiricists.

In epistemology and in its broadest sense, rationalism is ""any view appealing to reason as a source of knowledge or justification"" (Lacey, 286). In more technical terms it is a method or a theory ""in which the criterion of truth is not sensory but intellectual and deductive"" (Bourke, 263). Different degrees of emphasis on this method or theory lead to a range of rationalist standpoints, from the moderate position ""that reason has precedence over other ways of acquiring knowledge"" to the radical position that reason is ""the unique path to knowledge"" (Audi, 771).

In various contexts, the appeal to reason is contrasted with revelation, as in religion, or with emotion and feeling, as in ethics. In philosophy, however, reason is more often contrasted with the senses, including introspection but not intuition (Lacey, 286).

Since the Enlightenment, rationalism is usually associated with the introduction of mathematical methods into philosophy, as in Descartes, Leibniz, and Spinoza (Bourke, 263).

Rationalism is often contrasted with this view known as empiricism. Taken very broadly these views are not mutually exclusive, since a philosopher can be both rationalist and empiricist (Lacey, 286-287). Taken to extremes the empiricist view holds that all ideas come to us through experience, either through the five external senses or through such inner sensations as pain and pleasure, and thus that knowledge is essentially based on or derived from experience. At issue is the fundamental source of human knowledge, and the proper techniques for verifying what we think we know (see epistemology).

Proponents of some varieties of rationalism argue that, starting with foundational basic principles, like the axioms of geometry, one could deductively derive the rest of all possible knowledge. The philosophers who held this view most clearly were Baruch Spinoza and Gottfried Leibniz, whose attempts to grapple with the epistemological and metaphysical problems raised by Descartes led to a development of the fundamental approach of rationalism. Both Spinoza and Leibniz asserted that, in principle, all knowledge, including scientific knowledge, could be gained through the use of reason alone, though they both observed that this was not possible in practice for human beings except in specific areas such as mathematics. On the other hand, Leibniz admitted that ""we are all mere Empirics in three fourths of our actions"" (Monadology § 28, cited in Audi, 772).

Wikipedia contributors, 'Rationalism', Wikipedia, The Free Encyclopedia, 31 January 2007, 16:28 UTC, [accessed 31 January 2007]

For the philosophical origin of reductionism see the history of causality.