Halliday & Matthiessen (1999: 29):
Meaning is realised in two cycles. We have noted that the first cycle, the realisation in lexicogrammar, is natural, in the sense of being non-arbitrary: for example, the grammatical constituency structure of a clause provides a natural representation of the semantic configuration of a process, participants and circumstances. By attending to grammatical realisations, we can thus learn a good deal about the more abstract organisation of meaning at the higher stratum of semantics.