 Besides the ontological economy of only requiring inertial motion and its attendant force effects, Descartes’ choice of circularly moving bands of particles may have also been motivated by worries over, for lack of a better term, “plenum crowding”. In the Principles, he argues: “It has been shown…that all places are full of bodies…. From this it follows that no body can move except in a complete circle of matter or ring of bodies which all move at the same time” (Pr II 33). Circular motion is therefore necessary for Descartes because there are no empty spaces for a moving object to occupy. Although the world is described as “indefinitely” large (Pr I 26–27, with only God receiving the more positive description, infinite), the non-circular motion of a single body could violate the Cartesian conservation principle by resulting in an indeterminate material displacement. As an aside, it is enormously difficult to reconcile Descartes’ collision rules with his claim that all bodily motion occurs in circular paths; moreover, since the bodies that comprise the circular path all move simultaneously, it seems to follow from the definition of “body” (see  Section 3)  that there is only one moving body (and not many).