Counterparts and Actuality

David Lewis has argued that when it comes to interpreting claims about what might or must have been, we should use the resources of his counterpart theory, not those of quantified modal logic (Lewis 1968, Lewis 1986). Lewis has offered a host of reasons for this. The language of counterpart theory is purely extensional, and questions about logical relations between modal claims are reduced, via counterpart theory, to easily solvable questions about classical first-order logical consequence; and the language of counterpart theory has greater expressive power than the language of quantified modal logic—whatever can be expressed in the latter can be expressed in the former, but not vice versa. These two alleged benefits are not unique to counterpart theory, since they will be shared by any view of modality which takes as basic a non-modal first-order language, whether it mentions counterparts or not. But Lewis’s list of reasons for insisting on counterpart theory continues: only with the machinery of counterpart theory can we adequately explain the “inconstancy of representation de re,” the wavering back and forth of our intuitive judgments about essentialist claims; by appealing to counterpart theory we can solve problems of material constitution, without being committed either to the existence of distinct coincident objects or to the absurd view that identity is a relation that may hold only contingently between an object and itself; and counterpart theory permits us to endorse such haecceitistic-sounding claims as “I might have been Frank Sinatra while everything else is as it actually is,” without being committed to haecceitism proper, the view that there are distinct but qualitatively identical possible worlds. This is no small list of counterpart theory’s virtues, and many philosophers have been persuaded for these reasons that counterpart theory is preferable to quantified modal logic. We will argue, however, that even if the supposed virtues are genuine, 1 they are not worth their price: there is very good reason to reject