Halliday & Matthiessen (1999: 617):
Semiotically, the critical factor is that of metaphor; the semiotic bonds that had enabled the child to learn the mother tongue in the first place, bonds between figures and their elements on the one hand and clauses and their transitivity functions on the other, are systematically … untied. The categories of experience are deconstrued, to be recategorised … in the “objectifying” framework of grammatical metaphor.