Between the years 1643 and 1649, Princess Elisabeth of Bohemia (1618–80) and René Descartes (1596–1650) exchanged fifty-eight letters—thirty-two from Descartes and twenty-six from Elisabeth. Their correspondence contains the only known extant philosophical writings by Elisabeth, revealing her mastery of metaphysics, analytic geometry, and moral philosophy, as well as her keen interest in natural philosophy. The letters are essential reading for anyone interested in Descartes's philosophy, in particular his account of the human being as a union of mind and body, as well as his ethics. They also provide an insight into the character of their authors, and the way ideas develop through intellectual collaboration. Philosophers have long been familiar with Descartes's side of the correspondence. Elisabeth's letters add context and depth both to Descartes's ideas and the legacy of the princess. This annotated edition also includes Elisabeth's correspondence with the Quakers William Penn and Robert Barclay.