Halliday & Matthiessen (2014: 700):
There is thus a fundamental relationship between modal assessment, including modality, and projection. To bring this out, we can interpret modal assessment as interpersonal projection. Interpersonal projection always involves the speaker or addressee as ‘projector’: ‘I think’, ‘I say’; ‘do you think’, ‘do you say’. It is always implicit unless it is made explicit through grammatical metaphor, by ‘co-opting’ ideational resources to do interpersonal service.