Halliday & Matthiessen (1999: 233):
Lexical and grammatical metaphor are not two different phenomena; they are both aspects of the same general metaphorical strategy by which we expand our semantic resources for construing experience. The main distinction between them is one of delicacy. Grammatical metaphor involves the reconstrual of one domain in terms of another domain, where both are of a very general kind … Lexical metaphor also involves the reconstrual of one domain in terms of another domain; but these domains are more delicate in the overall semantic system.