Halliday & Matthiessen (1999: 532):
For a fuller understanding of the clause, we have to recognise that it evolved simultaneously as reflection, as action and as information: that is, not only as a representation of the phenomena of our experience but also as a means of social action, of moving around in interpersonal space (and so defining that space and those who occupy it); and as a semiotic construct, whereby language itself becomes a part of, and a metaphor for, the reality it has evolved to construe and to construct.