Halliday & Matthiessen (1999: 525):
If the ideational metafunction is language in its "third person" guise, the interpersonal is language in its "first and second person" guise; the interaction of a 'me' and a 'you'. The 'me' and the 'you' are of course constructed in language; they have no existence outside the social semiotic. Once constructed, me and you then become a part of experience and can be referred to alongside the him, the her and the it; but note that (unlike their interpersonal meaning, which does not change) their ideational meaning changes every time there is a change of speaker (this is what makes me and you so difficult for children to learn).