Halliday (2008: 147):
The grammar, in its ideational function, is a theory of human experience. And a very effective theory it is, because it has evolved along with ourselves, in what is now recognised to have been the co-evolution of language and the human brain. Constructing our model of reality along these particular lines has proved highly beneficial to our survival as a species on the planet. […] But this is how the grammar — the “content plane”, the lexicogrammar with the semantics — has evolved: it is the theory in terms of which we interpret and interact with our environment.