Halliday & Matthiessen (1999: 269):
We can say therefore that grammatical metaphor is predominantly a ‘nominalising’ tendency. But if we look at it semantically we can see that it is a shift from the logical to the experiential: that is, making maximum use of the potential that the system has evolved for classifying experience, by turning all phenomena into the most classifiable form — or at least into a form that is more classifiable than that in which they have been congruently construed.