 One common interpretation of the argument is that while one may have direct or privileged access to one's "current" mental states, there is no such infallible access to identifying previous mental states that one had in the past.That is, the only way to check to see if one has applied the symbol S correctly to a certain mental state is to introspect and determine whether the current sensation is identical to the sensation previously associated with S. And while identifying one's current mental state of remembering may be infallible, whether one remembered correctly is not infallible.Thus, for a language to be used at all it must have some public criterion of identity.