Halliday (2008: 157):
… there are various reasons why this metaphoric [nominal-based] kind of grammar is associated with writing while the congruent, clause-based grammar is associated with speech. The metaphoric grammar could not function as the primary construction of reality — “primary” in the sense of the child’s mother tongue, the grammar through which experience is first construed. This is because this grammar is interpretable only as a transformation of something else: only as metaphor, in fact. … This kind of grammar can only function as a secondary construal of experience — just as writing can only function as a secondary construal of meaning, because writing is also metaphoric to speech.