Halliday & Matthiessen (1999: 514):
Language itself, of course, is a form of human behaviour; “languaging” constitutes, for the grammar, another distinct type of process, that of “verbal” (or better, “symbolic”) processes. An act of saying is not simply externalising inner events; it is actively transforming them, into an event of a different kind. It then resembles other semiotic events, many of which do not require a conscious information source (… the light says stop).