Halliday & Matthiessen (1999: 17):
In what we have said so far we could seem to be taking an 'essentialist' or 'correspondence' approach to grammar and meaning, according to which 'meaning' preexists the forms in which it is 'encoded' (cf. Lakoff's, 1988, argument against the 'objectifying' view of meaning). In such a view, the grammar is said to be natural because it evolves to serve an already developed model of experience, a "real world" that has previously been construed.
In fact we are not taking an 'essentialist' or 'correspondence' approach, and there will be many places throughout our discussion where such an interpretation is clearly ruled out, as being incompatible with our own conception of semantics.
The view we are adopting is a constructivist one, familiar from European linguistics in the work of Hjelmslev and Firth. According to this view, it is the grammar itself that construes experience, that constructs for us our world of events and objects. As Hjelmslev (1943) said, reality is unknowable; the only things that are known are our construals of it — that is, meanings. Meanings do not 'exist' before the wordings that realise them. They are formed out of the impact between our consciousness and its environment.