The last work published by Descartes, the Treatise on the Passions of the Soul (1649) is the fruit of his entire philosophy. This treatise, which is based on a summary of Cartesian biology, moves towards a concrete medicine for psycho-physiological conditions and blossoms into an apology for generosity. To scientific observations, Descartes does not disdain to add psychological notations whose finesse sometimes evokes those maxims which flourished in salons in the 17th century. Thus the breadth of scientific, moral and metaphysical conclusions, ever-reviving sources of study for specialists, is additionally colored by a lived richness which makes this Treatise, written for a princess and offered to a queen, the model of works accessible to the general public.