Halliday & Matthiessen (1999: 503):
We have drawn attention at various points to the overall indeterminacy of language, something that we see as a necessary condition of its ability to function in the construal of experience. Our experience of the 'goings-on' in and around ourselves is so rich and many-faceted that no semiotic system that attempted to impose on it a rule-bound, determinate frame of reference would be 'functional' as a resource for survival.
We have tried to show such indeterminacy as a positive feature, and build it into our suggested meta-construal — not as some wayward or exceptional extravagance but as an unmarked state of affairs that is recognised to be the norm. To this extent, our grammatics becomes itself a metaphor for the grammar — that is, to the extent that we are able to enact this indeterminacy in our own representations.