Just as, according to Descartes, there had been no philosophy before his that rested on certain principles, so there could not have been any morality based on reason. That is why he “compared the moral writings of the ancient pagans to very proud and magnificent palaces built only on sand and mud”. Descartes’s ultimate philosophical ambition is thus to define a morality grounded on certain principles. Corresponding to different moments during the realization of this ambitious project are two distinct stages of Cartesian morality. Descartes first announces the rules of a morality that can be followed while one is seeking the foundations of knowledge, then formulates the precepts of his moral philosophy. This chapter examines that project.