ABSTRACT
The Cartesian universe consists of matter in motion. Introducing motion into extended substance (matter) divides it up into bodies, each with two primary modes that undifferentiated, undivided, and unlimited res extensa cannot have: shape and state of motion. Two contiguous parts of extended substance are only separated into distinct bodies by their relative motions; two contiguous parts are only joined in one body by their relative rest. Once in motion, bodies are constantly colliding with one another – in fact, collision is the only interaction they are capable of. The two primary modes of a body in turn have modes of their own: shapes have surfaces and motions have determinations. These are all quantitative concepts, and together with logic and Euclidean geometry they provide the basic conceptual tools that Descartes uses to construct a quantitative mechanistic physics. The deductive core of this physics, as presented in Part II of the Principia Philosophiae, is centred around the principle of the conservation of motion or force and three Laws of Nature that govern the behaviour of bodies. These laws describe how the force or motion of a body and the determination of this force regulate its behaviour in impact with other bodies.
