Consider this case: We have taught someone the use of the
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words “darker” and “lighter”. He could, e.g., carry out such an order as, “Paint me a patch of colour darker than the one I am shewing you.” Suppose now I said to him: “Listen to the five vowels a, e, i, o, u and arrange them in order of their darkness.” He may just look puzzled and do nothing, but he may (and some people will) now arrange the vowels in a certain order (mostly i, e, a, o, u). Now one might imagine that arranging the vowels in order of darkness presupposed that when a vowel was sounded a certain colour came before a man's mind, that he then arranged these colours in their order of darkness and told you the corresponding arrangement of the vowels. But this actually need not happen. A person will comply to the order: “Arrange the vowels in their order of darkness”, without seeing any colours before his mind's eye.