 The fact that compositionality—at least in the weak form it is usually stated and supported by arguments from productivity—does not require substitutivity is important. Substituting a complex expression for a simple one within some expression effects the structure of that expression, and since compositionality allows meaning to depend on structure, such a substitution can affect meaning. Consider one of Quine’s arguments; cf. (Quine 1953: 143). ‘It is necessary that the number of planets is greater than seven’ is false, ‘It is necessary that eight is greater than seven’ is true, even though ‘the number of planets’ and ‘eight’ have the same extension, so the semantics of English cannot be extensional.(Quine uses ‘nine’ because he thought Pluto was a planet.) The argument assumes that the extension of a definite description is whatever uniquely satisfies it—a claim rejected by proponents of quantificational accounts of definite descriptions (e.g. Russellians). But even if we grant this assumption as well as the compositionality of English, the argument still fails: the difference in truth-value could be the result of the different syntactic structure of the two sentences.