 Descartes’ correspondence with Princess Elisabeth has as its central topic the relation of mind and body—a relation that is explored from the point of view of both theory (the problem of mind-body union) and practice (see Shapiro 2007 for a full account of the correspondence). With regard to practice, Descartes is again concerned with the connection between physical and mental well-being, and in particular the deleterious effects of passions such as sadness, grief, fear and melancholy. He discusses these topics in the course of advising Elisabeth on how to cope with her own illness and distress. The question, however, is an ancient one: when faced with the hardships of life—physical illness, loss, anxiety—how can one respond in a way that allows one to preserve the tranquility that is the core of our happiness?