  Leibniz's critique of Descartes and his followers was focused principally on the Cartesian account of body or corporeal substance. According to Descartes, the essence of body is extension; that is, a corporeal substance is simply a geometric object made concrete, an object that has size and shape and is in motion. This view, indeed, is the cornerstone of the new mechanical philosophy to which Leibniz was originally attracted. Nevertheless, Leibniz came to see two distinct problems with this view. First, in claiming that the essence of body is extension, Descartes is endorsing the view that matter is infinitely divisible. But if matter is infinitely divisible, then one can never arrive at the simple unities that must exist at some ontological ground level. Second, if matter is simply extension, then there is in its nature no source of activity. If this is so, Leibniz thought, then the bodily objects of the world cannot count as substances.