Halliday & Matthiessen (1999: 444):
… all of our experience is construed as meaning. Language is the primary semiotic system for transforming experience into meaning; and it is the only semiotic system whose meaning base can serve to transform meanings construed in other systems (including perceptual ones) and thus integrate our experience from all its various sources.
Halliday & Matthiessen (1999: 509-10):
Language is set apart, however, as the prototypical semiotic system, on a variety of different grounds: it is the only one that evolved specifically as a semiotic system; it is the one semiotic into which all others can be “translated”; and it is the one whereby the human species as a whole, and each individual member of that species, construes experience and constructs a social order. In this last respect, all other semiotic systems are derivative: they have meaning potential only by reference to models of experience, and forms of social relationship, that have already been established in language. It is this that justifies us in taking language as the prototype of systems of meaning.