  Kaplan holds that since ‘I exist’ is valid, one could know a priori the proposition that it expresses, in a context. For example, if Mary considers and understands the sentence ‘I exist’, then she can know a priori the proposition that it expresses in her context. Thus Kaplan concludes that she can know a priori the proposition that she exists. But this proposition is contingent, so Mary can have a priori knowledge of a contingent proposition. Thus Kaplan claims that his logic of indexicals provides examples of Kripke’s (1980) contingent a priori truths. (For discussion, see Salmon 1991, Forbes 1989)