Halliday & Matthiessen (1999: 7):
One essential task for our semantics is that of modelling a particular phenomenon of the content plane that is known as grammatical metaphor. This is the phenomenon whereby a set of agnate (related) forms is present in the language having different mappings between the semantic and the grammatical categories, for example:
alcohol's dulling effect on the brainalcohol has a dulling effect on the brainalcohol has the effect of dulling the brainalcohol affects the brain by dulling itthe effect of alcohol is to dull the brainthe effect of alcohol is to make the brain dullif one takes/drinks alcohol it makes the brain dullif one takes/drinks alcohol the/one's brain becomes dull &c.
Since this phenomenon of grammatical metaphor is fundamental to adult uses of language, we shall take it as a central thrust of our book. One way in which we shall seek to demonstrate the validity and power of a semantic approach is by using it to handle grammatical metaphor, and to show how this pervasive aspect of the lexicogrammar expands the potential of the meaning base.