  The most natural diagnosis here is that the sentence’s meaning is determined by the meanings of its parts. We understand it because we understand its component words and because we understand the compositional mechanisms through which words combine to form meaningful sentences. Of course, word meanings may themselves be fixed by convention. But then what the conventionalist should explicate is the conventional link between words and their meanings, not just the conventional link between sentences and their truth-conditions. Lewis explicates the latter, not the former. Although Lewis (1992) attempts to circumvent these worries, Hawthorne (1993) and Schiffer (2006) argue that his response is inadequate.