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 multi-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.