Halliday & Matthiessen (2014: 256):
Within the general class of ‘mental’ clauses, there are four different subtypes of sensing: ‘perceptive’, ‘cognitive’, ‘desiderative’ and ‘emotive’. These are treated by the grammar as distinct types. They differ with respect to phenomenality, directionality, gradability, potentiality and ability to serve as metaphors of modality …