Halliday & Matthiessen (1999: 274, 275):
… the general phenomenon of metaphor, as an inherent property of language as a stratified semiotic, is a feature of the system as a whole — of the construal of meaning in lexicogrammatical terms.
… it is the potential for (grammatical) metaphor (itself a product of the stratified ideational resource system) that makes it possible to construe experience in terms of such complementarities and contradictions.