Halliday & Matthiessen (2004: 626):
Systemically, metaphor leads to an expansion of the meaning potential: by creating new patterns of structural realisation, it opens up new systemic domains of meaning. And it is the pressure to expand the meaning potential that in fact lies behind the development of metaphorical modes of meaning. Thus in the system of modality, the system of orientation is expanded by the addition of a systemic contrast in manifestation between ‘explicit’ and ‘implicit’…