Halliday & Matthiessen (1999: 274):
And we should stress once again that to describe the "reality" that is construed in this way as being generalised does not imply that it is "coherent", in the sense that it is internally consistent and unselfcontradictory. On the contrary: much of the power of metaphor derives from the tensions and contradictions set up
(a) within the metaphor itself,
(b) between one metaphor and another, and
(c) between the metaphor and other regions of the ideation base.