“But surely he used ‘lighter’ in a different sense when he said e was lighter than u”. ‒ ‒ What does this mean? Are you distinguishing between the sense in which he used the word and his usage of the word? That is, do you wish to say that if someone uses the word as he does, some other difference, say in
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his mind, must go along with the difference in usage? Or is all you want to say that surely the usage of “lighter” was a different one when he applied it to vowels?