Halliday & Matthiessen (1999: 603-4):
The central meaning–making resource of language — its “content plane” — is stratified into two systems: that of lexicogrammar, and that of semantics. The semantic system is the ‘outer layer’, the interface where experience is transformed into meaning. The ‘inner layer’ is the grammar, which masterminds the way this transformation takes place.
Halliday & Matthiessen (1999: 17):
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. … Meanings do not ‘exist’ before the wordings that realise them.