Halliday & Matthiessen (1999: 503):
We have tried to show that such indeterminacy [inherent in language] as a positive feature, and build it into our 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.