Mental Clause As Metaphorical Realisation Of Probability

Halliday & Matthiessen (2004: 614):

What’s happened here is that there has been a realignment in the realisational relationship between semantics and grammar. … a modalised proposition is realised as if it was a sequence, by a clause nexus of projection.  The effect is that the modality and the modalised proposition are separated, each being realised by a clause in its own right…