René Descartes’s medical studies compose an important section of his entire production. Yet, both their incomplete and fragmentary nature, along with several other problems make medicine a secondary field in Descartes’s philosophy, despite the fact he depicted it as a branch of the tree of his philosophy. While historians of philosophy have generally downplayed its importance, several scholars have recently attempted to restore the centrality of medicine in Descartes’s philosophical enterprise. Yet, Descartes also provided medicine with a specific use. In this article, I aim to disclose his philosophical uses of medicine, as three medicines surface in Descartes’s work. The first is the physiology of vision in L’Homme, which Descartes used to confirm the veracity of his physics (ca.1632-1633). The second is the pathology he used to confirm his metaphysics in the Meditationes de prima philosophia (1641). The third is the physiology of the composite he used to encompass the understanding of passion in Les Passions de l’âme (1649). While Descartes’s medical knowledge played a crucial architectural role in his philosophy, a narrowly focused medical enterprise surfaced, ultimately revealing the uses of medicine for the completion of his philosophical project.