  In his Principles of Philosophy, Descartes had argued that  the quantity of motion in the world remains constant, where the  quantity of motion of a body is determined by its speed times its  size. So, for example, at Principles 2:36, Descartes argues  that “if one part of matter moves twice as fast as another  which is twice as large, we must consider that there is the same  quantity of motion in each part; and if one part slows down, we must  suppose that some other part of equal size speeds up by the same  amount” (AT VIIIA 61/CSM 240). This general conservation  principle, which Descartes takes to be grounded in God’s immutable  nature, in turn serves as the foundation for his three laws of motion as well as his rules of impact. 