Halliday & Matthiessen (1999: 602):
… the way that language itself is organised, as a stratified, metafunctional system, recapitulates — acts out, so to speak — both the make–up of this environment in natural (physical–biological), social and semiotic systems–&–processes (our metafunctions) and the internal contradictions, complementarities and fractal patterning by which all such systems–&–processes are characterised (our stratification).