Leibniz argues that Cartesian extension lacks the unity required to be a substance. A key premise of Leibniz’s argument is that matter is a collection or aggregation. I consider an objection to this premise raised by Leibniz’s correspondent Burchard de Volder and consider a variety of ways that Leibniz might be able to respond to De Volder’s objection. I argue that it is not easy for Leibniz to provide a dialectically relevant response and, further, that the difficulty arises from Leibniz’s (...) commitment to part-whole priority in the case of material wholes, a commitment not shared by De Volder. One major implication is that Leibniz relies on a bottom-up conception of material things, which makes his argument vulnerable to objections stemming from certain types of monist positions. (shrink)