Indexical expressions like 'I,' 'now,' 'here,' 'that,' and tense change their reference depending on context—who is speaking, when, where, etc. The semantic content of 'I am hungry' depends on who is speaking. Context-sensitive expressions pose a challenge to truth-conditional semantics: the truth-condition of an indexical utterance cannot be specified without reference to context. Yet indexicals are central to natural language.
Analyze indexicals systematically: what do they refer to? How is reference determined? How do they interact with tense, modality, and other operators? Distinguish character (context-invariant rule) from content (reference fixed by context).
Indexicals are special or marginal—they are pervasive (all utterances involve a context of utterance). Context is merely psychological—context is partly objective (facts about speakers, times, places) and partly determined by the conversation.
From your work on meaning and reference, you know that the meaning of an expression is connected to what it picks out in the world. But some expressions work differently: their reference is not fixed by a stable descriptive content but shifts systematically with who uses them, when, and where. These are indexical expressions — "I," "you," "here," "now," "today," "this," and grammatical tense — and understanding them requires distinguishing two layers of meaning that a single expression carries simultaneously.
David Kaplan introduced the crucial distinction between character and content. The character of an indexical is its context-invariant rule for determining reference: the rule for "I" is always "the agent of the context" — whoever is speaking. This rule is the same no matter who uses "I." The content (or semantic value) is what the expression contributes to the proposition expressed on a particular occasion: when I say "I am hungry," the content is me specifically, not the general rule about speakers. Character is like a function; content is the output of that function applied to a particular context.
This two-level structure solves what would otherwise be a puzzle. "I am here now" is trivially true whenever uttered — but it expresses different propositions in different contexts (different speakers, places, and times). The sentence is not analytically true in virtue of meaning alone; rather, its character guarantees that any utterance of it will be true. Kaplan calls such sentences logically true in a special sense: true at every context of utterance, though the proposition expressed varies. Contrast this with "The president is at the White House," which has stable content but is not true at every context.
Context-sensitivity goes well beyond the classic indexicals. Many expressions — gradable adjectives like "tall" or "expensive," knowledge attributions, predicates of personal taste — have been argued to be context-sensitive in similar ways, with the context-invariant linguistic meaning underdetermining what proposition is expressed. This makes indexicality not a quirky side phenomenon but a window into the systematic relationship between linguistic meaning, context, and propositional content. The pragmatics/semantics boundary — your builds-toward topic — is partly defined by the question of which apparent context-sensitivity is built into linguistic meaning (semantics) and which is a matter of pragmatic enrichment after the fact.
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