  A third argument, especially important in connection with Leibniz’s  more general metaphysics, takes issue with the implication that  Cartesian bodies – or at least Cartesian matter taken as a  whole – might be on a metaphysical par with created minds.  Again, it might be helpful to see Leibniz’s thinking here as  proceeding in two steps. First, drawing on principles integral to his metaphysics, Leibniz insists that independence and  unity are marks of true substances. The independence  criterion suggests that we can usefully distinguish between created  substances that depend only upon God for their existence, and  derivative creatures that depend not only upon God but also upon the  existence of created substances as well. The unity criterion implies  a distinction between creatures that are true, indivisible unities,  and creatures that are mere accidental unities, or  “aggregates.” Leibniz maintains that only the former have a rightful claim to being genuine created substances, since, as he  famously puts it in a letter to Arnauld, “what is not truly  one being is not truly one being either” (G  II.97/AG 86).