For any pair of expressions that differ in reference, there exists some pair of sentences differing only by substitution of those expressions that differ in truth-value.

If this holds for sentences, does it also hold for subsentential expressions? It seems that it must. Suppose for reductio that two subsentential expressions, e and e*, have the same content but differ in reference. It seems plausible that two sentences which differ only by the substitution of expressions with the same content must have the same content. (While plausible, this principle is not uncontroversial; see entry on compositionality.) But if this is true, then sentences which differ only