Most Australian languages are commonly held to belong to the Pama–Nyungan family, a family accepted by most linguists, with Robert M. W. Dixon as a notable exception. For convenience, the rest of the languages, all spoken in the far north, are commonly lumped together as "Non-Pama–Nyungan", although this does not necessarily imply that they constitute a valid clade. Dixon argues that after perhaps 40,000 years of mutual influence, it is no longer possible to distinguish deep genealogical relationships from areal features in Australia, and that not even Pama–Nyungan is a valid language family.