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"Ludwig and Bertie were two precocious little tykes. Like many children, they played games with their own private languages. One of their favorites, which mystified the adults around them, was called 'Beetle'.
It started one day when they found two boxes. Ludwig proposed that they took one each, and that each would only ever look inside his own box, not that of the other. What is more, he would never describe what was in his box or compare it to anything outside the box. Rather, each would simply name the contents of his box 'beetle'.
For some reason, this amused them greatly. Each would proudly say that he had a beetle in his box, but whenever someone asked them to explain what this beetle was, they refused. For all anyone knew, either or both boxes were empty, or each contained very different things. Nonetheless, they insisted on using the word 'beetle' to refer to the contents of their boxes and acted as though the word had a perfectly reasonable use in their game. This was unsettling, especially for grown ups. Was 'beetle' a nonsense word or did it have a private meaning that only the boys knew?
Baggini, J., The Pig That Wants to Be Eaten, 2005, p. 67.
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At first glance, this hardly even seems to be a thought experiment, so I'd better add more from Baggini's explanation to show you why philosopher's concern themselves with such things. He says:
...all language use is a kind of game, in that it relies upon a combination of rules and conventions, not all of which can be explicitly stated, and which only players of the game really understand. ... Consider what happens when I say, "I have a pain in my knee." The box in this case is my inner experience. As with Ludwig and Bertie's containers, no one else can look inside it; only I can. ... All of the vocabulary of pain refers to sensations, and all of these are inside the boxes of our own subjective experience. ... For all we know, when we both say we are feeling pain, what is going on inside me is quite different from what is going on inside you.
What do you think? Have Ludwig and Bertie shown you our shared vocabularies aren't all that shared?