According to tranditional philosophical terminology and to most interpretations of Cartesianism, Descartes is a dualist. This dualism is expressed in his fundamental distinction between two substances—mind and matter—and, though admitted to be full of difficulties and by many to be untenable, it has very generally been regarded as at least a clearly intelligible doctrine, consistently held by Descartes. That this is not so has been shown by Professor Boyce Gibson in his able and careful analysis of Cartesianism. The aim of the present essay is to draw attention to another difficulty that has not been sufficiently noticed; whether it is an actual one or only an apparent one due to a misunderstanding of Descartes is a question that may be left to those who read him differently and who may be able to remove one more charge of inconsistency. The general contention is that Descartes—and this point is conceded in expositions of Descartes—seeks to prove that there are material bodies, that material bodies are necessary for his own account of the physical universe, and that yet his own theory of matter as extension, even combined with motion, makes the existence of bodies (in the plural) impossible and hence any proof of such existence impossible. In fact, the employment of the idea of motion only serves to emphasize the difficulty and inconsistency of the theory.