The French thinker René Descartes (1596-1650) is called the father of modern philosophy. He initiated the movement generally termed rationalism, and his Discourse on Method and Meditations defined the basic problems of philosophy for at least a century.

To appreciate the novelty of the thought of René Descartes, one must understand what modern philosophy, or rationalism, means in contrast to medieval, or scholastic, philosophy. The great European thinkers of the 9th to 14th century were not incapable of logical reasoning, but they differed in philosophic interests and aims from the rationalists. Just as the moderns, from Descartes on, usually identified philosophy with the natural and pure sciences, so the medievals made little distinction between philosophical and theological concerns.

The medieval doctors, like St. Thomas Aquinas, wanted to demonstrate that the revelations of faith and the dictates of reason were not incompatible. Their universe was that outlined...