  Some naturalists have explicitly relied on this motivation. For example, Lewis uses it to reject structuralism as the correct philosophy of set theory in Parts of Classes (1991, 58–9) even if he disclaims it as an argument; see also Colyvan (2001, 33), Shapiro (1997, 30) and Burgess (1998, 197). There is much to say about the argument, explored in Paseau (2005). Here we content ourselves with two critical observations.