Halliday & Matthiessen (1999: 614):
Since our concern in this book is with the ideation base, we have not been considering interpersonal aspects of meaning, and we have not put major emphasis on dialogic patterns in discourse. So in this final glance at ontogenesis we should foreground very clearly the fact that meaning is an interactive process and that children learning to mean construe their semiotic resources through dialogue. This is not simply an optional extra, something that makes the learning processes easier; it is an inherent property of semiosis itself.
Semiotic systems are social systems, and meaning arises in shared social consciousness; this is evident already in the protolanguage, when infants depend on being treated as communicating beings, and those within their "meaning group" are tracking them — unconsciously creating the language along with them (see Halliday, 1979b). We find this manifested also in the forms of discourse, in the way children participate in constructing narratives of shared experience (see Halliday, 1975: 112; Painter, 1989: 55).
When we talk of "construing experience" as the metafunctional realm of the ideation base, we are referring to the shared experience of the group, the culture and the species; it is by means of dialogue that children gain access to this shared experience and are enabled to construe their own experience with reference to it. And the dialogic nature of discourse serves the child also as a metaphor, as the semiotic manifestation of the social conditions of human existence.