Extensionality of Proper Part Containment

Abstract

Achille Varzi has shown that it is harder to deny Extensionality than we may have thought: he's argued that if we define proper parthood as parthood with distinctness, cases we take to violate Extensionality don’t really involve sharing of all proper parts. Aaron Cotnoir has responded by showing that, if we instead define proper parthood as asymmetric parthood, we can take Extensionality to be violated in these cases. I will offer a new response to this argument. Even granting Varzi's definition of proper parthood, there are versions of Extensionality very similar to the one Varzi and Cotnoir discuss. One of these is motivated by the same intuitions as Varzi's Extensionality, and is violated in cases traditionally taken to be anti-Extensional. Thus, even granting Varzi's definition of proper parthood, anti-Extensionalists can capture a sense in which their cases violate Extensionality, without this merely amounting to a violation of Uniqueness of Composition.