Halliday & Matthiessen (1999: 232):
The traditional approach to metaphor is to look at it 'from below' and ask what does a certain expression mean. For example the lexical metaphor flood means either, literally, an inundation of water or, metaphorically, an intense emotion as in she felt a flood of relief. But we could look 'from above' and ask how is intense emotion expressed. Then we would say it is expressed either, literally, as she felt very relieved or, metaphorically, as she felt a flood of relief. Once we look from above in this way, we can see that the phenomenon under discussion is the same as metaphor in its traditional sense except that what is varied is not the lexis but the grammar. Thus:Here in (a) the lexico-semantic domain of 'volume' has been mapped onto the lexico-semantic domain of 'meteorological commotion'; while in (b) the grammatico-semantic domain of 'figures' has been mapped onto the grammatico-semantic domain of 'participants'. The metaphoric principle is the same in both cases; they differ only in generality.
