Halliday & Matthiessen (1999: 184, 185, 186):
Grammatically speaking, (simple) participants are realised by nominal groups, which are made up of both things and qualities. In terms of the structure of the nominal group, the cut-off point between things and qualities is between the Classifier and the Thing: see Figure 5-5.
This distribution of qualities and things across the nominal group indicates two related points:(i) things are more time-stable than qualities; and(ii) things are more experientially complex than qualities. …
But this difference in experiential complexity is in turn related to the first of our two points, in that whatever is being construed as stable, as having persistence through time, is essentially a construct, an assemblage of different qualities, that (to borrow Jespersen's metaphor) can be crystallised only as an organic whole. The nominal group embodies this essential association between complexity and permanence.