The term rationalism (from the Latin ratio, "reason") has been used to refer to several different outlooks and movements of ideas. By far the most important of these is the philosophical outlook or program that stresses the power of a priori reason to grasp substantial truths about the world and correspondingly tends to regard natural science as a basically a priori enterprise. Although philosophies that fall under this general description have appeared at various times, the spirit of rationalism in this sense is particularly associated with certain philosophers of the seventeenth and early eighteenth centuries, the most important being René Descartes, Benedict de Spinoza, and Gottfried Wilhelm Leibniz. It is rationalism of this type that will be the subject of this entry.

Two other applications of the term should, however, be distinguished.

Rationalism in the Enlightenment

The term rationalism is often loosely used to describe an outlook allegedly...