Halliday & Matthiessen (1999: 136):
These can be treated as a subtype of material processes or as a borderline category between material and mental. They include conscious processing construed as active behaviour (watching listening, pondering, meditating) rather than just passive sensing (seeing, hearing, believing). Like the Senser in a mental clause, the ‘Behaver’ in a behavioural one is endowed with consciousness; whereas in other respects behavioural clauses are more like material ones. Like material clauses (but unlike mental ones), behavioural clauses can be probed with do: What are you doing? — I’m meditating but not I’m believing. Furthermore, behavioural clauses normally do not project, or project only in highly restricted ways (contrast mental: cognitive David believed —> the moon was a balloon with behavioural: David was meditating —> the moon was a balloon; nor can they accept a ‘fact’ serving as Phenomenon (mental: David saw that the others had already left but not behavioural: David watched that the others had already left). In these respects, behavioural processes are essentially part of the material world rather than the mental one. Many of them are in fact further removed from mental processes, being physiological rather than psychological in orientation.