Halliday & Matthiessen (1999: 264):
… the drift towards ‘thinginess’ is the culminating and most clearly articulated form of a shift which can be characterised in more general terms as a shift towards the experiential — towards that mode of construing experience that is most readily organised into paradigmatic sets and contrasts. Things are more easily taxonomised than qualities, qualities than processes, and processes more easily than circumstances or relations. Since the ‘noun-ness’ is being used to construe phenomena that start out as something else than a noun, metaphors will inevitably be abstract.