A separate article deals with a different philosophical position called rationalism.

Continental Rationalism is a school of philosophy based on the thesis that human reason is the source of knowledge. It originated with René Descartes and spread during the 17th and 18th centuries, primarily in continental Europe. In contrast, its contemporary rival, British Empiricism, held that all knowledge comes to us through experience or through our senses. At issue is the fundamental source of human knowledge, and what the proper techniques are for verifying what we think we know. (See Epistemology.)

Note, however, that the distinction between Rationalists and Empiricists was drawn much later, and would not have been recognised by the philosophers involved. Also, the distinction wasn't as clear-cut as is sometimes suggested; for example, the three main Rationalists were all committed to the importance of empirical science, and in many respects the Empiricists were closer to Descartes in their methods and metaphysical theories than were Spinoza and Leibniz. Thus, although it can be useful when organising courses or writing books, the distinction is less useful philosophically.

Rationalists argued that starting with intuitively-understood basic principles, like axioms of geometry, one could deductively derive what was true. Descartes, with his mathematical background, was naturally drawn toward this method, and famously claimed to derive knowledge of his own existence from the fact that he was thinking (cogito, ergo sum) — though in fact it's more accurate to say that he considered each person's knowledge of her own existence to be an immediate intuition, in no need of argument or proof. On the heels of his work came continental philosophers such as Spinoza and Leibniz whose attempts to grapple with the epistemological and metaphysical problems raised by Descartes led to a development of the fundamental approach of Rationalism.

Spinoza and Leibniz both thought that, in principle, all knowledge – including scientific knowledge – could be gained through the use of reason alone, though they both accepted that in practice this wasn't possible for human beings except in specific areas such as maths. Descartes, on the other hand, was closer to Plato, thinking that only knowledge of eternal truths – including the truths of mathematics, and the epistemological and metaphysical foundations of the sciences – could be attained by reason alone; other knowledge required experience of the world, aided by the scientific method.

Immanuel Kant started as a Rationalist, but after being exposed to David Hume's works which "awoke [him] from [his] dogmatic slumbers", Kant arguably synthesized the Rationalist and Empiricist traditions.

The more modern usage of the term "Rationalist" refers to the belief that human behaviour and beliefs should be based on reason. See rationalism.