  Leibniz’s ingenious attack on this Cartesian model of qualitative  variety proceeds in two steps. The first step charges that motion  alone is unable to account for qualitative variety at an instant:  since all qualitative variety in the Cartesian system depends on  motion, and there is no motion in an instant, it follows that in a  Cartesian world there could be no qualitative variety at an   instant.[12]  The second step of Leibniz’s argument charges that if the world is  qualitatively homogenous at every instant, then it must be  qualitatively homogenous over time as well. For if the world is  qualitatively undifferentiated at each instant, then every instant  will be qualitatively identical, and so the world as a whole will not undergo any qualitative change as it passes from one instant to the  next. To use an anachronistic analogy, the two steps taken together  imply that a Cartesian world would be like a filmstrip whose every  frame is blank, and thus whose projection would not only be  homogenous at each instant, but through time as well.