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...