The difficulty which we express by saying “We can't know what he sees when he (truthfully) says that he sees a blue patch” arises from the idea that “knowing what he sees” means:
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“seeing that which he also sees”; not however in the sense in which we do so when we both have the same object before our eyes: but in the sense in which the object seen would be an object, say, in his head, or in him. The idea is that the same object may be before his eyes and mine, but that I can't stick my head into his (or my mind into his, which comes to the same) so that the real and immediate object of his vision becomes the real and immediate object of my vision, too. By “I don't know what he sees” we really mean “I don't know what he looks at”, where “what he looks at” is hidden and he can't show it to me; it is before his mind's eye. Therefore, in order to get rid of this puzzle, examine the grammatical difference between the statements “I don't know what he sees” and “I don't know what he looks at”, as they are actually used in our language.