Halliday & Matthiessen (2014: 259):
As we have seen, ‘material’ clauses are concerned with our experience of the material world and ‘mental’ clauses are concerned with our experience of the world of our own consciousness. Both this outer experience and this inner experience may be construed by ‘relational’ clauses; but they model this experience as ‘being’ rather than ‘doing’ or ‘sensing’.