Contexts, Demonstratives, and Validity

In his seminal work, “Demonstratives”, David Kaplan shows how to define validity (or logical truth) for a language containing indexical expressions such as ‘I’, ‘now’, ‘today’, ‘that’, and others. Kaplan’s great insight in that work is that the indices for context and for circumstances of evaluation (which Kaplan takes to be time and world) play conceptually distinct roles in semantics. To capture these distinct roles, Kaplan distinguishes not only between the intension of an expression and the extension of the expression at a particular circumstance of evaluation, but also between the character of an expression and the content of the expression relative to a particular context. Identifying content with intension, Kaplan posits three distinct levels of semantic evaluation: character, content, and extension. In Kaplan’s terminology, character combines with context to generate content, and then content combines with a circumstance of evaluation to generate an extension (such as the referent of a singular term or the truth value of a sentence). Given this notion of content, Kaplan is able to give a systematic account not only of truth relative to a context for sentences containing indexical expressions, but also of logical truth. Assuming that a context uniquely determines a circumstance of evaluation, he defines a sentence to be true relative to a context just in case the content of that sentence relative to that context applied to the time and world of the context returns the value True. Finally, Kaplan defines a sentence to be a logical truth (or to be valid) just