 In the Principles, Descartes conservation law only recognizes a body’s degree of motion, which correlates to the scalar quantity “speed”, rather than the vectorial notion “velocity” (which is speed in a given direction). This distinction, between speed and velocity, surfaces in Descartes’ seven rules of impact, which spell out in precise detail the outcomes of bodily collisions (although these rules only describe the collisions between two bodies traveling along the same straight line). Descartes’ utilization of the concept of speed is manifest throughout the rules. For example: