Halliday & Matthiessen (2014: 249):
In a clause of ‘mental’ process, there is always one participant who is human; this is the Senser, introduced above: the one that ‘senses’ – feels, thinks, wants or perceives, for example, Mary in Mary liked the gift. More accurately, we should say human-like; the significant feature of the Senser is that of being ‘endowed with consciousness’. 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.