Halliday (2008: 140):
Writing is, as I said, parasitic on spoken language; it could not have come into existence except as a surrogate of speech. But it is not, and never has been, just the same thing in a new guise. Writing is not just spoken language written down, unlike the visual system of deaf sign (“sign language”), which is truly a variant of spoken language and shares the functions and fluidities of speech. […] This is to look at writing “from above”, in terms of its meaning and its function in human cultures.