Halliday & Matthiessen (1999: 525):
Note in this connection that the basic distinction constructed by language is not, as sometimes claimed, that between 'me' and 'the rest'; it is that between 'me-&-you' on the one hand, and the rest — the 'third party' — on the other. This distinction is coded in the grammar at many places, for example in the system of modality; there can be no "first person" unless there is a "second person" with whom these roles can be alternately acted out.