 Astonishingly, Descartes claims that a smaller body, regardless of its speed, can never move a larger stationary body. While obviously contradicting common experience, the fourth collision rule does nicely demonstrate the scalar nature of speed, as well as the primary importance of quantity of motion, in Cartesian dynamics. In this rule, Descartes faces the problem of preserving the total quantity of motion in situations distinguished by the larger body’s complete rest, and thus zero value of quantity of motion. Descartes conserves the joint quantity of motion by equipping the stationary object C with a resisting force sufficient to deflect the moving body B, a solution that does uphold the quantity of motion in cases where C is at rest. That is, since B merely changes its direction of inertial motion, and not its size or degree of speed (and C equals zero throughout the interaction), the total quantity of motion of the system is preserved. For Descartes, reversing the direction of B’s motion does not alter the total quantity of motion, a conclusion that is in sharp contrast to the later hypothesis, usually associated with Newton and Leibniz, that regards a change in direction as a negation of the initial speed (i.e., velocity). Thus, by failing to foresee the importance of conjoining direction and speed, Descartes’ law falls just short of the modern law for the conservation of momentum.