Universalism doesn’t entail extensionalism

Abstract In the literature on mereology it is often accepted that mereological universalism entails extensionalism. More precisely, many accept that, if parthood is assumed to be a partial order (and, thus, the relevant theory of parthood is taken to be at least as strong as ‘core mereology’), the thesis that every plurality of entities has a mereological fusion entails the thesis that different composite entities have different proper parts. Central to this idea is the principle known as ‘Weak Supplementation’ which many take to impose an important constraint on the relation of proper parthood. In this paper I argue that this claim is false as the principle that I will call ‘Minimal Supplementation’ appears to be capable of doing all the work done by Weak Supplementation but without entailing extensionalism if conjoined with universalism and core mereology.