Lewis's pragmatic theory of the a priori is given a partial defense. Though the strictly logical function may be vulnerable to Quinean objections, there is an epistemological function of the a priori that is not. Lewis's use of Einstein on simultaneity has problems, but correcting for those errors does not affect the basic idea. Lewis's theory of the pragmatic a priori is then contrasted with the similar ideas in Arthur Pap, who Lewis influenced, as well as Michael Friedman and Hasok Chang.