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 grammatical categories … 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.