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.