This study combines historical research and philosophical analysis to cast light on why and how Cartesianism failed as a complete metaphysical system. After an initial discussion of methods in the history of philosophy, there is an analysis and criticism of late-17th-century Cartesianism, a survey of Cartesian theology, and two analytic chapters on Cartesian metaphysics which aim to demonstrate its logical inconsistency. The author argues that Descartes' ontology is incoherent and vacuous, his epistemology deceptive and his theology unorthodox - indeed, that "Descartes knows nothing".