Halliday & Matthiessen (1999: 7-8):
The content plane of a natural language is functionally diverse: it extends over a spectrum of three distinct modes of meaning, ideational, interpersonal and textual. These highly generalised functions of the linguistic system are referred to in our theory as metafunctions. The ideational metafunction is concerned with construing experience — it is language as a theory of reality, as a resource for reflecting on the world. The interpersonal metafunction is concerned with enacting interpersonal relations through language, with the adoption and assignment of speech roles, with the negotiation of attitudes, and so on — it is language in the praxis of intersubjectivity, as a resource for interacting with others. The textual metafunction is an enabling one; it is concerned with organising ideational and interpersonal meaning as discourse — as meaning that is contextualised and shared. But this does not mean processing some preexisting body of information; rather it is the ongoing creation of a semiotic realm of reality.