Halliday & Matthiessen (2014: 698):
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. Speakers being what we are, however, we like to give prominence to our own point of view; and the most effective way of doing that is to dress it up as if it was this that constituted the assertion (‘explicit’ I think ... ) – with the further possibility of making it appear as if it was not our point of view at all (‘explicit objective’ it’s likely that ... ).