Ontologically, a body (Latin corpus; French corps) is nothing but extension, its principal attribute, just as a mind is nothing but thought (AT VIIIA 25, CSM I 210). As the famous wax experiment of the Second Meditation shows, a body is not the same as its color, smell, shape, or motion, but instead merely something “capable of being extended” in many different ways (AT VII 31, CSM II 21). Not even solidity, in the sense of felt hardness, pertains to the essence of body since a body would retain its nature even if it always receded from our touch (AT VIIIA 42, CSM I 224). Solidity in the sense of impenetrability does pertain to the essence of body since penetration implies co-location and co-location implies annihilation: “But what is annihilated does not penetrate anything else” (AT V 342, CSMK 372). Besides being extended, bodies are by nature divisible and quantifiable, but Descartes emphasizes that divisibility (AT IXB 53, CSM 1 215n1) and quantity (AT VIIIA 44, CSM 1 226) are themselves merely conceptually distinct from extension and hence from body itself. Finally, since our strict notion of space (what Descartes calls “internal place”) is also simply extension, it follows that “in reality the extension in length, breadth, and depth which constitutes a space is exactly the same as that which constitutes a body” (AT VIIIA 45, CSM I 227). Consequently, a vacuum or “empty space” is impossible (AT VIIIA 49, CSM I 229–30), and the body comprising the universe is unlimited and unified (AT VIIIA 52, CSM I 232). So “body considered in general,” or generic matter, consists simply in “being something which is extended in length, breadth and depth” (AT VIIIA 42, CSM I 224).

For the purposes of first philosophy, it does not concern Descartes that this austere, geometrical conception of body is far removed from the diverse phenomenology contained in our familiar experience of objects. And yet the purpose of natural philosophy is to explain the regular phenomena of nature from first principles and laws. So Descartes needs to bridge the gap between body in general and natural bodies. Unlike the parts of a purely geometrical space, there are intrinsic, persistent differences among natural bodies. And associated with these differences are diverse but regular behaviors.