Halliday & Matthiessen (1999: 602):
In other words, language has evolved as part of our own evolution. It is not arbitrary; on the contrary, it is the semiotic refraction of our own existence in the physical, biological, social and semiotic modes. It is not autonomous; it is itself part of a more complex semiotic construct — which, as we have tried to show, can be modelled in stratal terms such that language as a whole is related by realisation to a higher level of context (context of situation and of culture). This contextualisation of language, we suggested, was the critical factor which made it possible to relate language to other systems-&-processes, both other semiotic systems and systems of other kinds.