Halliday & Matthiessen (1999: 187):
We have observed that things tend to be relatively stable in space and time, and relatively complex in their semantic make-up and in their interrelationships; and that these special characteristics of things are construed linguistically in various ways. By comparison with other elements, things tend to stand out
(i) by their varied roles as elements in figures,(ii) by the overall weight and discursive force of their primary categorisation of experience,(iii) by their tendency to be elaborated into numerous micro-categories,(iv) by their complex internal organisation, and(v) by their highly systematic relationship one with another.