Halliday & Matthiessen (1999: 237, 238):
Why then in our present interpretation have we to recognise two parts, one a lexicogrammar and one a semantics? Because the system continued to evolve beyond that point, enriching itself (i.e. engendering a richer model of experience) by forcing apart the two 'facets' of the sign so that each could take on a new partner — sequences could be realised by other things than clause complexes, processes could be realised by other things than verbs, and so on. …It is this step that gives rise to grammatical metaphor. When a sequence is realised as a clause complex, or a process as a verb, this is congruent: it is the clause complex, and the verb, in the function in which it evolved. When a sequence is realised as something other than a clause complex, or a process as something other than a verb, this is metaphorical. Some other grammatical unit is supplanting them in these functions.