Related to the alleged circularity of the definitions of motion and body, as well as the problem of resting bodies, is the difficulty in reconciling Descartes’ definition of “substance” with his claim that individual bodies are substances. If, as Descartes believes, substances are not dependent on other things in order to exist (Pr I 51), then any part of extension (which is a body, via Pr II 10, as explained above) would not qualify as a substance since it depend on its contiguous neighbors to