Now one may go on and ask: “How do you know that he has got toothache when he holds his cheek?” The answer to this might be, “I say, he has toothache when he holds his cheek because I hold my cheek when I have toothache”. But what if we went on asking:– “And why do you suppose that toothache corresponds to his holding his cheek just because your toothache corresponds to your holding your cheek?” You will be at a loss to answer this question and find that here we strike rock bottom, that is we have come down to conventions. (If you suggest as an answer to the last question that, whenever we've seen people holding their cheeks and asked them “what's the matter”, they have answered, “I have toothache”, ‒ ‒ ‒ remember that this experience only co-ordinates holding your cheek with saying certain words.)