 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 delimit and define its boundary. Yet, Descartes often declares that individual bodies are substances; e.g., “the two halves of a portion of matter, no matter how small they may be, are two complete substances” (AT III 447). One of the most popular replies to this difficulty, from Spinoza (Ethics, Part I, Prop. 15) to many contemporary commentators (e.g., Keeling 1968, Lennon 1993, Sowaal 2004, Schmaltz 2020, and numerous others), is to declare that only the whole plenum is a substance, and not any of its constituent parts. The problem with this attempted solution, however, is that it lacks textual support, as is evident in the Pr I 51 quotation above. Likewise, some of these reconstructions, such as Lennon’s, would seem to violate central aspects of Cartesian physics and metaphysics, for he interprets motion as a phenomenal contribution of the mind, such that the plenum and its parts do not move or change at all. Along these same lines, some scholars (e.g., Schaffer 2009) have concluded that Descartes was a supersubstantivalist, i.e., the view that space (spacetime, in the modern setting) is the only predicable or fundamental substance. While Descartes’ identification of corporeal substance and space (see above, Pr II 10) might seem to support this reading, supersubstantivalism takes space as primary, and matter as secondary or derived from space (see Sklar 1974, 222). Descartes, on the contrary, takes matter or body as primary and space as a derived, abstract concept: “the same extension which constitutes the nature of body also constitutes the nature of space, and . . . these two things differ only in the way that the nature of the genus or species differs from that of the individual” (Pr II 11). Whereas space is a genus or species concept for Descartes (which is a universal; Pr I 59), space is the individual for the supersubstantivalist, and thus ascribing supersubstantivalism to Descartes violates his nominalism (Pr II 8). Indeed, the reason that Descartes seeks to equate bodily and spatial extension in this part of the Principles is that he strives to reject any view that treats space as a separate, usually incorporeal, entity that is independent of matter (e.g., the popular imaginary space tradition, which was a forerunner to the absolutist or substantivalist conception): “That corporeal substance, when distinguished from its quantity or extension, is confusedly conceived as if it were incorporeal” (Pr II 9).