"Ludwig and Bertie were two precocious little tykes. Like many children, they played games with their own private languages. One of their favourites, which mystified the adults around them, was called 'Beetle'.
It started one day when they found two boxes. Ludwig proposed that they take 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.
This little story, which (as one of my readers said) hardly even seems like a thought experiment, was originally meant simply to introduce us to the idea of a "private language," just so it could then be rejected. In this case, Ludwig and Bertie have their own private language, wherein "beetle" clearly just means something like "my secret space", and this is kept hidden from the adults around them. But since the two tykes each understand this, their language is technically still "public." What Wittgenstein goes on to investigate though, is whether or not a single person can actually have a truly private language. For his argument, a private language must be "incapable of translation into an ordinary language - if, for example, it were to describe those inner experiences supposed to be inaccessible to others. ... A private language must be unlearnable and untranslatable, and yet it must appear that the speaker is able to make sense of it."
But such a speaker couldn't make any sense of it. As pointed out in the the Stanford Encyclopaedia of Philosophy, "a language in principle unintelligible to anyone but its originating user is impossible. The reason for this is that such a so-called language would, necessarily, be unintelligible to its supposed originator too, for he would be unable to establish meanings for its putative signs." In other words, a private language would cease to be a language at all. Language, therefore, is social.
Why does this matter? As discussed in the Wikipedia entry on the private language argument, "this would have profound implications for other areas of philosophical and psychological study. For example, if one cannot have a private language, it might not make any sense to talk of private experiences or of private mental states." We must be careful here not to conflate (1) ideas that are private because they are *hidden* from others, with (2) ideas that are private because they *cannot* be shared with others. We are trying to concern ourselves with whether or not there is privacy in the second sense, with whether or not there are unsharable aspects of consciousness.
To put a finer point on this, it helps to know that Wittgenstein introduced his beetle in a box during his investigations of the nature of pain. "Pains occupy a distinct and vital place in the philosophy of mind for several reasons. One is that pains seem to collapse the appearance/reality distinction. If an object appears to you to be red it might not be so in reality, but if you seem to yourself to be in pain you must be so: there can be no case here of seeming at all. At the same time, one cannot feel another person’s pain, but only infer it from their behavior and their reports of it. By offering the "beetle" as an analogy to pains, Wittgenstein suggests that the case of pains is not really amenable to the uses philosophers would make of it."
I think the phenomenon of phantom limb pain also calls into question whether philosophers can use pain to collapse the distinction between reality and how it appears to you, but Wittgenstein's beetle story is a more direct challenge to his mentor Bertrand Russell. It was Russell who most openly described the use of a private language in the second of his published lectures, ‘The Philosophy of Logical Atomism’, where he says: "In a logically perfect language, there will be one word and no more for every simple object, and everything that is not simple will be expressed by a combination of words, by a combination derived, of course, from the words for the simple things that enter in, one word for each simple component. A language of that sort will be completely analytic, and will show at a glance the logical structure of the facts asserted or denied. … A logically perfect language, if it could be constructed, would not only be intolerably prolix, but, as regards its vocabulary, would be very largely private to one speaker. That is to say, all the names that it would use would be private to that speaker and could not enter into the language of another speaker." Although Wittgenstein does not specifically call out his mentor and this description of a perfect language as private, it is considered likely that this is the target of his beetle box argument.
So the beetle box in itself is a deceptively small argument placed into a larger discussion by Wittgenstein of attempts "to clarify some of the problems involved in thinking of the mind as something over and above behaviour. Wittgenstein is trying to point out that the beetle is very much like an individual’s mind. No one can know exactly what it is like to be another person or experience things from another’s perspective (look in someone else’s box), but it is generally assumed that the mental workings of other people’s mind are very similar to our own."
From the perspective of an evolutionary philosophy, this is highly self-evident. For other philosophers to claim that our internal thoughts and feelings are ineffable, unknowable, and "private" from others in society, is to deny the billions of years of evolutionary history that we share, during which time the (essentially) same bodily structures were created everywhere in our species as we evolved to survive in the shared environment we exist within in this one universe. As neuroscientists unravel the functions of our brain structures, we don't find infinite varieties of beetles (or non-beetles) crawling around in our heads; we find 99.5% similarities in our molecular sub-structure. We are not so alone in our minds...even if other's thought experiments can sound awfully confusing at first blush.