Halliday & Matthiessen (1999: 510):
Meanwhile what has become clear is that there is the (by now) familiar interplay between phenomenon and observer. Wehave talked of physical, biological, social and semiotic systems as being categories of phenomena — which in an important sense they are. But they may also be thought of as different stances taken by the observer; thus we find physical and biological systems being interpreted as semiotic systems, in a kind of intellectual game which turns out to reveal new aspects of physical and biological processes. It is obviously beyond our scope — and indeed beyond our capabilities — to pursue these matters here. But they add a whole new dimension to our grammatics, to the concept of a theory of grammar as a metatheory of human experience.
On the other hand, construals of experience as physical and biological systems are semiotic systems: language supported by other semiotic modes made possible by language. As such, the models can be reconstrued in terms of linguistic theory.