The Passion of the Soul

A Mathematician Writes About Feelings

Picture a man at his wife's funeral. His face is wet, his chest heaves, and everyone around him sees a husband undone by grief. Yet Descartes asks us to imagine something stranger. In the deepest part of that same man's soul, a quiet joy may be stirring at the very moment his body weeps. The sadness is genuine. So is the joy. They sit side by side, one belonging to the body's commotion, the other to something the soul has worked out for itself. That single image — sorrow and gladness lodged in one person at once — is the puzzle this book sets out to solve.

It is an odd subject for René Descartes. He made his name with geometry and with a method of doubt so severe that he was willing to question whether the world existed at all. He is the man who decided he could be sure of only one thing to start with: that he was thinking. So why would a philosopher of pure reason spend his final book on the wet, messy, irrational business of human emotion?

The short answer is a princess. In the years before he wrote this, Descartes carried on a long correspondence with Elisabeth of Bohemia, a sharp and persistent thinker who kept pressing him on the question he least wanted to face. If the mind is one kind of thing and the body is a completely different kind of thing, how on earth do they touch? How does a decision, which has no weight and takes up no space, move an arm made of flesh? And how does a stubbed toe, which is pure flesh, produce the felt sting of pain in something immaterial? Elisabeth would not let it go, and to his credit, neither would he. *The Passions of the Soul*, published in 1649, just months before he died in the cold of a Swedish winter, is in large part his attempt to give her an honest answer.