Halliday (2008: 165-6):
As written language evolved, itself as a metaphor for language in its original, spoken form, it opened the way to the metaphoric reconstrual of experience: to new ways of meaning, the forms of educational and technical knowledge that constitute the semiotic aspect of a set of massive changes in the human condition. This is where we find the complementarity, not of speech and writing as material modes (states of matter), but of spoken and written language as modes of meaning. The metaphorical and the congruent are complementary states of meaning — because speaking and writing both shape reality in their own image. Each one makes the world look like itself.