Rationalism is a method of inquiry that regards reason as the chief source and test of knowledge and, in contrast to empiricism, tends to discountenance sensory experience. It holds that, because reality itself has an inherently rational structure, there are truths--especially in logic and mathematics but also in ethics and metaphysics--that the intellect can grasp directly. In ethics, rationalism relies on a "natural light," and in theology it replaces supernatural revelation with reason.
The inspiration of rationalism has always been mathematics, and rationalists have stressed the superiority of the deductive over all other methods in point of certainty. According to the extreme rationalist doctrine, all the truths of physical science and even history could in principle be discovered by pure thinking and set forth as the consequences of self-evident premises. This view is opposed to the various systems which regard the mind as a tabula rasa (blank tablet) in which the outside world, as it were, imprints itself through the senses.
The opposition between rationalism and empiricism is, however, rarely so simple and direct, inasmuch as many thinkers have admitted both sensation and reflection. Locke, for example, is a rationalist in the weakest sense, holding that the materials of human knowledge (ideas) are supplied by sense experience or introspection, but that knowledge consists in seeing necessary connections between them, which is the function of reason.
Most philosophers who are called rationalists have maintained that the materials of knowledge are derived not from experience but deductively from fundamental elementary concepts. This attitude may be studied in Ren Descartes, Gottfried Wilhelm Leibniz, and Christian von Wolff. It is based on Descartes's fundamental principle that knowledge must be clear, and seeks to give to philosophy the certainty and demonstrative character of mathematics, from the a priori principle of which all its claims are derived. The attack made by David Hume on the causal relation led directly to the new rationalism of Kant, who argued that it was wrong to regard thought as mere analysis. In Kant's views, a priori concepts do exist, but if they are to lead to the amplification of knowledge, they must be brought into relation with empirical data.
Excerpt from the Encyclopedia Britannica without permission.