Halliday & Matthiessen (1999: 6-7):
Thus we can say that a grammatics is a theory of grammar, while a grammar is (among other things) a theory of experience. But to show that a grammar is a theory of experience we use a functional, semantically motivated grammatics, since this allows us to seek explanations of the form of the grammar in terms of the functions to which language is adapted. But this closeness of fit between the semantics (i.e. the meaning) and the grammar does not mean that our grammatics can take over the semantic domain. Adopting a functional approach enables us to extend the domain of grammar in significant ways in the direction of semantics — not thereby reducing the scope of the semantics but rather enabling us to investigate how experience is construed in semantic terms — to develop the "meaning base" model that is the topic of the present book.