Halliday & Matthiessen (1999: 236-7):
In the congruent form the pattern of realisation is as follows:Looking at these from the standpoint of the evolution of language, when we say they are the congruent forms we are claiming not merely that they evolved first but that this is why they evolved. One of the contexts in which grammar came into being — one of its metafunctions — was that of construing human experience; and, as we have seen, the model that emerged was one which construed the continuum of goings-on into taxonomies: taxonomies of parts (meronymic) and taxonomies of kinds (hyponymic). The central construct was that of the 'figure'; figures could be further constructed into 'sequences' and also deconstructed into 'elements'.
How did the grammar construe this hierarchy of phenomena? — as clauses, clause complexes, and elements in the structure of the clause:The elements making up a figure were of three kinds: a process, participants in that process, and circumstantial features. How did the grammar construe this classification? — as verbs, nouns, and other things:The circumstance could be either some quality of the process or some participant that was indirectly involved:
