Does Universalism Entail Extensionalism?

Does a commitment to mereological universalism automatically bring along a commitment to the controversial doctrine of mereological extensionalism —the view that objects with the same proper parts are identical? A recent argument suggests the answer is ‘yes’. This paper attempts a systematic response to the argument, considering nearly every available line of reply. It argues that only one approach—the mutual parts view—can yield a viable mereology where universalism does not entail extensionalism.