Halliday & Matthiessen (2014: 251-2):
In a ‘macrophenomenal mental’ clause, the Phenomenon is realised by a non-finite clause denoting an act; … An act is a configuration of a process, participants involved in that process and possibly attendant circumstances … Macrophenomenal Phenomena are typically restricted to one subtype of ‘mental’ clause — clauses of perception: the act is seen, heard, tasted or perceived in some other way; but it is not normally thought, felt emotionally, or desired.