Halliday & Matthiessen (2004: 624):
The explicitly subjective and explicitly objective forms of modality are all strictly speaking metaphorical, since all of them represent the modality as being the substantive proposition. Modality represents the speaker’s angle, either on the validity of the assertion or on the rights and wrongs of the proposal; in its congruent form, it is an adjunct to a proposition rather than a proposition in its own right.