  Kaplan’s semantics entails that that propositions can vary in truth-value from one time to another. He also assumes that propositions are the things that agents assert and believe. But he admits (1989a, 503, n. 28) that the objects of assertion and belief are traditionally thought to be eternal, that is, not to vary in truth-value from time to time. Salmon (1989) argues that Kaplan’s theory needs to be amended so as to make contents eternal. Salmon argues that once this amendment is made, it becomes evident that to deal adequately with tense, a semantic theory needs both (a) content-like meanings that vary in extension from time to time in the way that Kaplan’s contents do and (b) content-like meanings that are eternal. Jeffrey King (2003) also criticizes Kaplan for allowing contents (propositions) to be non-eternal. See Cappelen and Hawthorne (2009) for related discussion.