Understanding Classical Mechanics: A Dialogue with the Cartesian Theory of Motion

We make an excursus to the history of mechanics and consider the set of laws of motion established by Rene Descartes in the seventeenth century, followed by his inferred rules of collisions of material bodies. This knowledge preceded the mechanics of Newton who was inspired by Descartes’ theory. Observing the history in retrospect, the laws of Descartes may be seen as including important ideas—the uniform rectilinear motion as a natural state and the concept of quantity of motion (momentum). Descartes stated conservation of momentum in interaction (collisions); however, because Descartes’ momentum was defined incorrectly, his account of collisions was incorrect, violating Galileo’s principle of relativity which implies rest-uniform motion equivalence. We analyze Descartes’ laws of motion and each of his rules for elastic collisions, revealing the specific logic of Descartes and show how the following scholars—Wallis, Wren, Huygens, and Newton—refined some and refuted other of his statements regarding the conservation of momentum. Finally, we compare the laws of motion obtained by Descartes with those of Newton. The critique of Descartes’ theory is both ontological and epistemological. It emphasizes the inappropriate neglecting of empirical verification in favor of principles postulated as certain and convincing.