Halliday & Matthiessen (1999: 291):
… the metaphorical dimension in grammar is in reality a cline. There are often numerous intermediate steps between the “most congruent” and the “most metaphorical” wordings; indeed it is the scale of metaphoricity that is reasonably clearly defined, not its end points. Given two agnate wordings that are positioned along this scale, we seldom have any difficulty in locating them relative to each other: we know which of the two is the more metaphorical one. But we [would be] hard put to it to specify a point at either end where we feel we could go no further.