Halliday & Matthiessen (1999: 528):
… and if we observe children developing their mother tongue we can see how the ideational and interpersonal resources of the system gradually emerge from the earliest semiotic encounters, in a way which may plausibly mimic how the metafunctions originally evolved. The textual metafunction is different because it does not originate in an extrinsic context of this kind. Rather, it is intrinsic to language itself.