A detailed scholarly biography of one of the intellectual founders of the modern world, by a distinguished French scholar, the second after Stephen Gaukroger's 1995 ``intellectual biography.'' Every educated person knows Descartes's one famous line (``cogito, ergo sum''); but the full story of what he thought and who he was is less familiar. Rodis-Lewis, professor emeritus at the Sorbonne and a winner of the Grand Prix of the AcadÇmie Franáaise, undertakes to repair that deficiency. Born in 1596, Descartes was the son of a well-to-do lawyer. His training in mathematics and philosophy came at the remarkably egalitarian Jesuit-run College of La Fleche (in Paris); even at that stage, he insisted on finding things out for himself, and read widely in subjects outside the normal curriculum, including alchemy and astrology. Rodis-Lewis often disagrees with previous biographers on the effect of these readings on his mature philosophy. At age 21, he joined the army (the Thirty Years War was just beginning), hoping to see the world; and for most of the next two decades, Descartes returned to France only rarely, living primarily in the Netherlands. His voluminous correspondence made him a familiar figure in the intellectual circles of the time. In due course, he revolutionized not only philosophy (with his ground-breaking Discourse on Method) but mathematics (contributing largely to the invention of calculus). He died in 1650, at the court of Queen Christina in Sweden. Rodis-Lewis gives the reader the broad pattern of Descartes's life, but she is primarily interested in the origins and development of his thought, and uses both his correspondence and his various journals to trace the sparks for his major ideas and intellectual preoccupations. A good deal of the text is spent in quibbling with previous biographers over dates. And she makes very little concession to the interests of readers who are not thoroughly grounded in the history of the period and of its ideas. Balanced and well documented, this work will be of interest primarily to historians and scholars.