  Leibniz’s second argument focuses on the ability of Cartesian physics to account for qualitative variety in the world. The thesis that the  whole essence of matter is extension blocks Descartes from holding  that matter is itself intrinsically diverse as well as from supposing that matter is differentially distributed in empty space. Clearly  recognizing the constraints imposed by his reductive account of body, Descartes elegantly maintains that all qualitative variety is to be  grounded in the motion of bodies, that is, that “All the  variety in matter, all the diversity of its forms, depends on  motion” (Principles 2:23/CSM 1:232). Qualitative variety  amongst bodies for Descartes thus appears to be grounded not in the  intrinsic natures of bodies or their parts, but rather in the way in  which the parts of bodies move relative to one another. To take an  example friendly to Descartes, it might be supposed that the  qualitative differences between a block of ice, a puddle of water,  and a cloud are to be explained not by appeal to intrinsic  differences in the elements that constitute them, nor by appeal to  the density of their elements in space, but rather in terms of the  relative speeds holding between those elements. One might thus  imagine – incorrectly of course – that the hardness of  ice depends only on water molecules being at rest with respect to one another, the fluidity of water only on water molecules moving with  respect to one another at a moderate speed, and the etherealness of a cloud only on water molecules moving with an extreme relative speed. 