 Yet another important source of externalist proclivities in twentieth century philosophy lies in the thought that the meaningfulness of a speaker’s utterances depends on its potential intelligibility to hearers: language must be public—an idea that has found varying and influential expression in the work of Ludwig Wittgenstein, W.V.O. Quine, and Donald Davidson. This, coupled with the assumption that intentionality (or “thought” in the broad (Cartesian) sense) must be expressible in language, has led some to conclude that what determines the content of one’s mind must lie in the external conditions that enable others to attribute that content.