Rationalism (Latin ratio,”reason”), in philosophy, a system of thought that emphasizes the role of reason in obtaining knowledge, in contrast to empiricism, which emphasizes the role of experience, especially sense perception.

Rationalism has appeared in some form in nearly every stage of Western philosophy, but it is primarily identified with the tradition stemming from the 17th-century French philosopher and scientist René Descartes. Descartes believed that geometry represented the ideal for all sciences and philosophy. He held that by means of reason alone, certain universal, self-evident truths could be discovered, from which the remaining content of philosophy and the sciences could be deductively derived. He assumed that these self-evident truths were innate, not derived from sense experience. This type of rationalism was developed by other European philosophers, such as the Dutch philosopher Baruch Spinoza and the German philosopher and mathematician Gottfried Wilhelm Leibniz. It was opposed, however, by British philosophers of the empiricist tradition, such as John Locke, who believed that all ideas are derived from the senses.

Epistemological rationalism has been applied to other fields of philosophical inquiry. Rationalism in ethics is the claim that certain primary moral ideas are innate in humankind and that such first moral principles are self-evident to the rational faculty. Rationalism in the philosophy of religion is the claim that the fundamental principles of religion are innate or self-evident and that revelation is not necessary (see Deism). Since the end of the 1800s, however, rationalism has played chiefly an antireligious role in theology.