RenĂ© Descartes (1596â€“1650) is one of the towering and central figures in Western philosophy and mathematics. His apothegm â€śCogito, ergo sumâ€ť 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 Descartesâ€™s death, Gottfried Leibniz, inventor of calculus and one of the greatest mathematicians in history, moved to Paris in search of this notebookâ€”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 pagesâ€”which, though written in code, he amazingly deciphered there on the spot. Leibnizâ€™s hastily scribbled notes are all we have today of Descartesâ€™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.