Halliday & Matthiessen (1999: 263-4):
It is possible to distinguish two predominant motifs in the phenomena characterised here: one major or primary and one minor or secondary one.
(i) The primary motif is clearly the drift towards ‘thing’.
(ii) The secondary motif is what appears as a tendency in the opposite direction: the move from ‘thing’ into what might be interpreted as a manifestation of ‘quality’ (qualifying, possessive or classifying expansions of the 'thing').