Halliday & Matthiessen (1999: 135):
Nature of participants: In a mental clause, the Senser is endowed with consciousness — s/he thought the moon was a balloon but not normally it thought the moon was a balloon. This constraint does not apply to any of the participants in material or relational clauses. While the Senser is heavily restricted in this way, the other mental participant, the Phenomenon, is entirely unrestricted: it can be not only phenomenal (she remembered the old house) but also macro-phenomenal (act: she remembered him coming down the stairs) or metaphenomenal (fact: she remembered that they had been happy in the old house). Participants in a material process cannot be metaphenomenal. For instance, while it is possible to demolish not only concrete things such as buildings but also abstract things such as ideas and arguments (she demolished the house/their ideas/his argument), it is not possible to demolish "metathings" (we do not find she demolished that the earth was flat).