 As a consequence of his first law of motion, Descartes insists that the quantity conserved in collisions equals the combined sum of the products of size and speed of each impacting body. Although a difficult concept, the “size” of a body roughly corresponds to its volume, with surface area playing an indirect role as well. This conserved quantity, which Descartes refers to indiscriminately as “motion” or “quantity of motion”, is historically significant in that it marks one of the first attempts to locate an invariant or unchanging feature of bodily interactions. To give an example, if a body B of size 3 and speed 5 collides with a body C of size 2 and speed 4, then the total quantity of motion of the system is 23, a quantity which remains preserved after the collision even though the bodies may possess different speeds.