External and internal private language arguments.

The external version of the PLA (private language argument) argues that there can be no such thing as x using "k" meaningfully unless it is logically possible for any other person y to acquire reason to believe that x is doing so. The internal version claims that it must be logically possible for x himself to acquire reason for belief in the meaningfulness of his own usage. Both versions of Wittgenstein's argument are here set out explicitly, and defended against various objections that have been made over the last forty years.