Halliday (2008: 168):
Thus the complementarity of speaking and writing is not a case of reciprocity. They do different things; not the same thing in different ways. Writing is secondary, derived; because of this it reaches a higher level of abstraction, creating through semantic junction a kind of semiotic stability, both ideational and interpersonal. Ideationally, it raises the commonsense theory of experience on to a more abstract and explanatory plane; so proverbs are replaced by axioms and equations as repositories of human knowledge. Interpersonally, it creates distance, and interaction among strangers: virtual interaction, as well as virtual dialogue brought about by projection. So writing is at one and the same time both more constraining and more enabling than speaking; and the fullest semiotic potential of the human brain, as we have inherited it, requires both.