Halliday & Matthiessen (1999: 15):
We have mentioned two corollaries of using a functional grammar. One was that the grammar is 'natural': that is, the forms of the grammar are non-autonomous, non-arbitrary in their relation to meaning. The other was that the grammar is explained in functional terms: specifically, in systemic grammar, in terms of the interaction among various functional constructions of meaning or 'metafunctions': ideational, interpersonal, and textual.