Let us group together clauses of feeilng, thinking and perceiving under the general heading of MENTAL PROCESS [...].
- Halliday (1985), Pag. 107

In a clause of mental process, there is always one participant who is human; this is the one that ʻsensesʼ – feels, thinks, or perceives, for example 'Mary' in 'Mary liked the gift'. More accurately, we should say human-like; the significant feature of such a participant is that of being ʻendowed with consciousnessʼ. Expressed in grammatical terms, the participant that is engaged in the mental process is one that is referred to pronominally as 'he' or 'she', not as 'it' [...] With regard to the other main element in a clause of mental process namely that which is felt, thought or perceived, the position is in a sense reversed. That is to say, the set of things that can take on this role in the clause is not only not restricted to any particular semantic or grammatical category, it is actually wider than the set of possible participants in a material process. It may be not only a ʻthingʼ but also a ʻfactʼ.
- Halliday (1985), Pag. 108