Halliday & Matthiessen (1999: 234):
… lexical metaphors have grammatical implications: they occur at the lexical degree of delicacy in the overall system, but precisely because grammar and lexis form a continuum related by delicacy, lexical domains are in fact more delicate elaborations of grammatical ones. So, for example, if understanding is construed metaphorically as grasping, it follows that a high degree of understanding can also be construed according to the same material model: understand very well => grasp firmly, as in she grasped the principles firmly. Similarly, if intensity is construed metaphorically in terms of location or movement in abstract space, this lexical reconstrual also has grammatical consequences, e.g. in terms of circumstantial elements within metaphorical figures: prices fell sharply, prices rose to a new high, costs hit the ceiling, and so on.