Universalism and extensionalism revisited

Abstract We present a new notion of mereological sum that is inequivalent to extant ones in the literature and does not fall prey to reasonable complaints that can be raised against some such notions. In light of this notion, we then revisit the relation between mereological universalism and extensionalism. In particular we argue that Varzi’s claim to the point that universalism entails extensionalism is justified only insofar as one sticks to Varzi’s notion of sum. In effect, we distinguish different versions of extensionalism and argue that universalism—when cashed out in terms of our new notion of sum—entails some versions but not others. Most significantly it does not entail extensionality of proper parthood. In the light of the above we set forth a new mereological system, Universalist Quasi-Supplemented Mereology, that can be considered a worthy alternative to different mereological systems in the literature.