Halliday & Matthiessen (1999: 613-4):
… you need a semiotic of a different kind, one that allows for a purely abstract level of representation “in between” the two faces of the sign. To put this another way, the sign has to be deconstructed so that, instead of content interfacing directly with expression, the relationship is mediated by a systematic organisation of form (a lexicogrammar). In other words, the semiotic has to become stratified.