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 towards 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.
We saw in discussing have a look, make a mistake &c. that if you make look, mistake into nouns you can expand them within nominal groups: have another good long look, don't make the same silly spelling mistake again! We now have classes of mistake (spelling mistake); properties, both experiential (long look) and interpersonal (silly mistake), quantities, and identities (that same mistake, another look, three mistakes). The same principle holds when any process is reconstrued metaphorically as a thing.