René Descartes (1596–1650) is one of the towering and central figures in Western philosophy and mathematics. His apothegm “ Cogito, ergo sum ” marked the birth of the mind-body problem, while his creation of so-called Cartesian coordinates have made our physical and intellectual conquest of physical space possible. But Descartes had a mysterious and mystical side, as well. Almost certainly a member of the occult brotherhood of the Rosicrucians, he kept a secret notebook, now lost, most of which was written in code. After Descartes’s death, Gottfried Leibniz, inventor of calculus and one of the greatest mathematicians in history, moved to Paris in search of this notebook—and eventually found it in the possession of Claude Clerselier, a friend of Descartes. Leibniz called on Clerselier and was allowed to copy only a couple of pages—which, though written in code, he amazingly deciphered there on the spot. Leibniz’s hastily scribbled notes are all we have today of Descartes’s notebook, which has disappeared. Why did Descartes keep a secret notebook, and what were its contents? The answers to these questions lead Amir Aczel and the reader on an exciting, swashbuckling journey, and offer a fascinating look at one of the great figures of Western culture.

Often called the father of modern philosophy, René Descartes set the intellectual agenda for seventeenth-century philosophy, mathematics, natural science, and beyond. In this critical biography, based on compelling new research, Steven Nadler follows Descartes from his early education in France to the Dutch Republic, where he lived most of his adult life, to his final months as a tutor to Queen Christina of Sweden. Along the way, Nadler shows how Descartes renewed philosophy by transforming fundamental assumptions about the cosmos, natural world, and human nature as well as how his work continues to generate new insights into many of the metaphysical and epistemological problems that engage philosophers today.