Les Passions de l'âme (The passions of the soul) is a treatise on moral philosophy, published in Paris in 1649, in which the philosopher René Descartes (1596−1650) theorizes on “the passions,” or what contemporary readers would call emotions. Descartes argues that passions are a matter of nature and therefore of the body. They are not inherently bad for the spirit, as long as they are kept in check by morals and free will, which are capable of evaluating the passions. While Descartes continues in a long tradition of theorizing about the passions, his psychological approach is groundbreaking and foreshadows the birth of neurophysiology as a scientific discipline. In the context of the mechanistic view of life that was gaining popularity in 17th century science, Descartes perceived the body as an autonomous machine; hence his physiological approach to the passions of the soul. Passions no longer are perceived as illnesses, but rather as natural manifestations, the workings of which Descartes seeks to explain. Descartes had been exchanging letters with Princess Elisabeth of Bohemia about various philosophical subjects, one of which was the question of morals. This treatise on the passions of the soul is the fruit of that correspondence.