This dissertation situates Berkeley's "master argument" for idealism (PHK I, 23 and DHP I, 200) in the context of Descartes' theory of ideas, and seeks to show that within that context the argument is convincing. In addition, the dissertation argues that Descartes' theory of ideas was not representationalist., as is often supposed, but a kind of direct realism; Cartesian ideas render intelligible individuals directly present to the intellect. In this respect Cartesian idea theory is very similar to a theory of species expounded by Antonio Rubio and other Jesuit philosophers at the turn of the seventeenth century; Jesuit writings of this period include several interesting anticipations of Cartesian doctrine. Finally, the dissertation discusses the relationship between Berkeley's master argument and the semantic paradoxes of Berry, Koenig and Richard, and suggests that all these arguments commit a fallacy of vicious circularity, related to but distinct from the fallacy signaled by Bertrand Russell.