Halliday & Matthiessen (2004: 636):
The interpersonal metafunction defines the environment in which children first learn the strategy of grammatical metaphor [a manifestation of a more general principle]. No doubt this is partly because interpersonal metaphors tend to make selections more explicit, as when probability is realised by a ‘mental’ clause projecting the modalised proposition (‘explicit’ orientation), and partly because the interpretation of interpersonal metaphors is often both supported and ‘tested’ immediately in the ongoing dialogic interaction.