 While Descartes’ first and second laws deal with the rest and motion of individual bodies, the third law of motion is expressly designed to reveal the properties exhibited among several bodies during their collisions and interactions. In short, the third law addresses the behavior of bodies under the normal conditions in his matter-filled world; when they collide: “The third law: that a body, upon coming in contact with a stronger one, loses none of its motion; but that, upon coming in contact with a weaker one, it loses as much as it transfers to that weaker body” (Pr II 40). In the following sections of the Principles, Descartes makes explicit the conserved quantity mentioned in this third law: