We have surveyed Wittgenstein's reflections on the nature of language and linguistic representation, and we have looked at the destructive consequences of his conception for metaphysics. Now it is time to move on to what has rightly been said to be the most important philosophical argument of the twentieth century. It is known as ‘the private language argument’, and it occurs in Investigations §§243–315. It might better have been dubbed ‘the private language arguments’ – in the plural – since there is not one argument but a whole battery of bewilderingly interwoven ones. It links Wittgenstein's philosophy of language with his philosophy of psychology.

Wittgenstein had an uncanny ability to dig down deeper into the presuppositions of our thought than any other philosopher. It is a common philosophical failing, he said, not to put the question marks deep enough down. In the case of the Private Language Arguments, Wittgenstein himself put the question mark so deep down that at first glance it is exceedingly difficult to see why the question he raises matters in the slightest. Here is how he begins:

So, the question Wittgenstein addresses is whether there could be a radically private language. A private language, as he uses the phrase, is not a language which, as it happens, others cannot understand (such as Esperanto, before its inventor Zamenhof, went public).