Grammatical structure underdetermines semantic content; pragmatic principles and contextual factors must narrow interpretation. This explains why context sensitivity is ubiquitous in natural language and why the same sentence can express different propositions in different contexts.
Study cases of genuine underdetermination where syntax alone cannot determine meaning. Distinguish syntactic ambiguity from pragmatic underdetermination to appreciate the difference.
From your study of truth conditions, you know that a sentence's meaning can be captured by specifying the conditions under which it is true. And from the semantics/pragmatics boundary, you know that what a sentence means (semantics) and what a speaker communicates by using it (pragmatics) can come apart. Semantic underdetermination brings these two insights together in a sharp way: even the literal, conventional meaning of a sentence — its semantic content — often underdetermines what proposition it expresses, and context must supply the missing content.
Consider "It is raining." This is grammatically complete and syntactically unambiguous. But what proposition does it express? Rain somewhere? Rain here, now? Rain in the location relevant to the conversation? The sentence doesn't specify a time or place — those parameters are implicit and must be filled in contextually. The truth-conditional content of the sentence is incomplete without context. This is not mere ambiguity (where a word has two dictionary meanings) and not mere implicature (where the literal content is enriched by conversational inference). It is a case where the logical form itself leaves open a variable that context must saturate.
Semantic underdetermination is pervasive in natural language. "I've had enough." (Enough of what?) "Everyone left." (Everyone in the universe, or everyone contextually salient?) "Steel is stronger than plastic." (Stronger how, under what conditions, in what configurations?) These sentences all require contextual supplementation before they express a determinate, evaluable proposition — before we can even ask whether they are true or false. Stanley, Sperber and Wilson, and others have mapped the various mechanisms: free enrichment (context adds content with no grammatical slot to receive it), saturation of implicit arguments (grammar posits a variable that context fills), and loosening (context shifts the truth-conditional boundaries of predicates).
The philosophical significance is this: the traditional picture of semantics as fully autonomous — sentences having complete truth conditions fixed by grammar and lexicon alone — turns out to be false for most ordinary utterances. The boundary between what is semantically encoded and what is pragmatically supplied is not clean. This challenges strong semantic minimalism (the view that sentences have minimal truth conditions independent of context) and supports contextualism: the view that context shapes not just what is conversationally communicated but what is literally expressed. Understanding underdetermination is prerequisite to evaluating the semantics-pragmatics debate, indexicality, and the proper scope of formal semantic theories.
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