Halliday & Matthiessen (2004: 626):
…the semantic domain of modality is extended through grammatical metaphor to include explicit indications of subjective and objective orientation: a modal proposition or proposal is realised, as if it were a projection sequence, by a nexus of two clauses, rather than by a single clause. Here the modal assessment itself is given the status of a proposition in its own right; but because the projecting clause of the nexus is metaphorical in nature, standing for an interpersonal assessment of modality, it is also, at the same time, a modal Adjunct in the clause realising the proposition/proposal. This is the general effect of grammatical metaphor: it construes additional layers of meaning and wording.