Halliday & Matthiessen (1999: 536):
Many "spoken" languages may be realised in two different media: in speech, and in writing. At first this presents itself just as two modes of expression; but when we look more closely at discourse in spoken and written language we find regularly associated differences in grammatical construction. We find written language constructed in nominal groups, whereas spoken language is typically constructed in clauses. And since it is in the grammar that our experience is construed into meaning, what we are seeing are different forms of the construction of experience, one couched primarily in terms of figures, the other in terms of elements that make up such figures, mainly those that function as participating entities.
Our basic approach to this is embodied in the term "metaphor", as used in the context of metaphor in the grammar. We used the expression "grammatical metaphor" to refer to a complex set of interrelated effects whereby, in English and many other languages, there have evolved what seem to be alternative representations of processes and properties, in terms of word classes, meanings prototypically construed as verbs or adjectives come to be construed as nouns instead. But, as we saw, this is simply the superficial manifestation of a wider and deeper phenomenon affecting the entire construal of experiential meanings in the grammar.