Halliday & Matthiessen (2014: 261):
… while one participant in a ‘mental’ clause, the Senser, is always endowed with consciousness, this is not the case with ‘relational’ clauses. If anything, the participants in ‘relational’ clauses are more like the Phenomenon of a ‘mental’ clause – not only things, but also acts and facts can be construed as participants in a ‘relational’ clause.