Consider this case: someone says it's green| I can't understand it I see everything blue today & vice versa. We answer: it must look queer! He says it does & e.g. goes on to say how cold the glowing coal looks & how warm the clear (blue) sky. I think we should under these or similar circumstances be inclined to say that he saw red what we saw green. And again we should say that we know that he means by the words blue & red what we do as he has always used them as we did|had.