  Some scholars emphasize the epistemological aspects of Descartes' work, starting with the Rules and continuing through to the Principles. Accordingly, the main change in Descartes' intellectual development is the introduction of skeptical arguments in the Discourse and Meditations. Many interpreters, represented prominently in the latter twentieth century by Richard Popkin (1979), believe that Descartes took the skeptical threat to knowledge quite seriously and sought to overcome it in the Meditations. By contrast, in the main interpretive thread  followed here, skeptical arguments were a cognitive tool that Descartes used in order to guide the reader of the Meditations into the right cognitive frame of mind for grasping the first truths of  metaphysics. Achieving stable knowledge of such truths would have as a side-effect security against skeptical challenge.