Abstract
This chapter defends Saul Kripke's argument. The basic principle underlying the argument is that if a sentence contains only terms that pick out their referents essentially then, if the sentence expresses a necessary truth, it is impossible for people in our epistemic situation to be mistaken in asserting the sentence. It seems clear that people in our epistemic situation might be mistaken in asserting the sentences that materialists assert as necessary truths. The option available to materialists is to claim that physicalist terms do not pick out their referents essentially. But that leads to an implausible view of physical reality akin to idealism.