Between 1698 and 1706 Leibniz was engaged in one of his most interesting correspondences, with the Dutch philosopher and physicist Burcher de Volder. The two men were concerned primarily with the question of how the motion of bodies can be explained without appeal to the direct intervention of God. Leibniz presented a naturalistic account of motion to De Volder, but failed to convince him of its adequacy. I shall examine one reason for this failure - the disagreement that arose over (...) the issue of whether there is a substance whose nature is constituted by extension. (shrink)