Les passions de l’âme, Descartes’ last major work, was published in November 1649, shortly before his death. This work grew out of his correspondence with Princess Elisabeth of Bohemia during the 1640s. The correspondence opens with Elisabeth urging Descartes to explain “how the soul of a human being (in being only a thinking substance) can determine the bodily spirits, in order to bring about voluntary actions” (May 6, 1643; AT III 661). Elisabeth's genuine theoretical interest in the mind-body interaction was accompanied by a more practical concern: crowning Descartes “the best doctor for my soul,” she expected him to cure her body by supplying her with remedies for the afflictions of her soul (May 6, 1643; AT III 662; May 24, 1645; AT IV 208). This twofold initiative prompted Descartes to develop his theory of the passions and related ethics in a new, systematic fashion.

The Passions of the Soul opens with Descartes conveying unease with the ancients’ teachings on the passions. He is therefore “obliged to write just as if [he was] considering a topic that no one had dealt with before.” In a prefatory letter to the treatise, he proclaims his intention “to explain the passions only as a natural philosopher [physicien], and not as a rhetorician or even as a moral philosopher” (AT XI 326, CSM I 327). This claim may seem puzzling given the weight he assigns in this work to the ethical ramifications of the analysis of the passions. In underestimating his significant ethical concerns, Descartes may have intended to stress the novelty of his own mechanistic approach to the topic. On his approach, the passions are mental phenomena originating in bodily changes, which are transmitted to the pineal gland through movements of the animal spirits without the concurrence of the will. Departing from the Aristotelian Scholastic accounts of the passions as phenomena belonging to the appetitive faculty, Descartes seeks to provide a new mechanistic account of the passions and their physiological origin, in accordance with his new physics and the metaphysical dualism of body and mind.