Although the proper formulation and assessment of Ludwig Wittgenstein's argument (or arguments) against the possibility of a private language continues to be disputed, the issue has lost none of its urgency. At stake is a broadly Cartesian conception of experiences that is found today in much philosophy of mind.

What Is a Private Language?

In §243 of Philosophical Investigations (1967; see also §256) Wittgenstein introduces the idea of a language in which "a person could write down or give vocal expression to his inner experiences—his feelings, moods, and the rest—for his private use. … The individual words of this language are to refer to what can only be known to the person speaking; to his immediate private sensations. So another person cannot understand the language." In subsequent sections (according to some commentators, up to as far as §315) Wittgenstein criticizes the possibility of such a...