Halliday & Matthiessen (1999: 141):
Both cognition and desideration can come to serve as metaphors for the interpersonal system of modality — for modalisation and modulation respectively — alongside congruent realisations such as modal auxiliaries and adverbs. That is, a number of processes of cognition can stand for probabilities — I think : probably, I suppose : perhaps; and a number of processes of desideration can stand for inclinations and obligations — I want : should, I insist : must. For instance:I think that in a sense you’ve had to compromise, haven’t you?Neither perceptive nor emotive sensing can serve as metaphors for modalities.