Facing the Dark Side of Twin Earth

This account is motivated almost exclusively by its capacity to explain our judgment about XYZ. Since H2O and XYZ do not differ in their superficial characteristics, Twin Earth seems to show that natural kind terms are indexical and that natural kinds are individuated by their causally relevant deep structure. These two claims constitute what I will call the Minimal Putnamian Account (MPA). There appears to be a general consensus that MPA must be true if our judgment about XYZ stems from overarching principles. It thus seems that an independent refutation of MPA and the many proposals that subsume it would suggest that whatever judgments we make about such contrived cases as Twin Earth should be taken as noise and not data to be explained. Sharing in this impression, some have tried to undermine the externalist thesis by criticizing MPA on the basis of imaginative thought experiments that suggest that natural kinds are not individuated by their deep structure (for example, Bach [1987], Mellor