Halliday & Matthiessen (1999: 205-6):
The discussion of 'facets' illustrates the difference between things and qualities of things; it also illustrates the indeterminacy of the distinction. Let us begin our discussion of qualities by relating them to things.
[Previously] we discussed the complementary contributions made by qualities and things in the construal of participants. We showed that qualities and things differ in two related respects, temporal stability and experiential complexity: things tend to persist through time and to represent intersections of many dimensions, whereas qualities tend to be less stable through time and tend to represent values on single dimensions. Qualities thus construe values on dimensions such as size (e.g., 'big/ small'), weight ('heavy/light'), and shape ('round'/ 'square'/ 'rectangular'/ 'oval'...).