Halliday & Matthiessen (1999: 602):
Language is able to create meaning because it is related to our material being (ourselves, and our environment) in three distinct and complementary ways.
In the first place, it is a part of the material world: the processes of language take place in physiological (including neural) and physical space and time.
In the second place, it is a theory about the material world: language models the space-time environment, including itself, in a "rich" theoretical mode: that is, both construing it (our ideation base) and enacting it (our interaction base).
In the third place, it is a metaphor for the material world: 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).
In other words, language has evolved as part of our own evolution.
Blogger Comments:
To be clear, this characterises the material world as a construal of experience as meaning, such that language itself is part of that semiotic construal, language is a theory about that semiotic construal, and language is a metaphor for that semiotic construal. That is, language creates meaning out of prior meaning: that created by perceptual systems.