Halliday & Matthiessen (1999: 135-7, 136n):
As with all systems in language, any given instance will be more or less prototypical; and there may be subtypes lying intermediately at the borderline of the primary types. The grammar construes the non-discreteness of our experience by creating borderline cases and blends. One such area is that of behavioural processes (Halliday, 1985: 128-9): "processes of physiological and psychological behaviour, like breathing, dreaming, smiling, coughing". These can be interpreted 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 as 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);² 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.
Such borderline cases, in which the pattern of reactances does not conform exactly to that of a major type, are typical of grammatical systems in general.
² For the special case of quoting by a behavioural process, as in "You're late again", she frowned, cf. the discussion of saying in Section 4.3.
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Caveat: Importantly, this is Matthiessen, not Halliday. For Halliday, behavioural processes do not project. Instead, instances like David was meditating (that) the moon was a balloon are mental clauses, where a behavioural feature has been added to a mental Process. Likewise, instances like "You're late again", she frowned are verbal nexuses, where a behavioural feature has been added to a verbal Process. Importantly, process type depends on the clause in which a verbal group functions, not on the most probable function of a verb (meditating, frowned).