Mental Clause As Metaphorical Realisation Of Probability

Halliday & Matthiessen (2004: 614):

…the probability is realised by a mental clause as if it were a figure of sensing.  Being metaphorical, the clause serves not only as the projecting part of a clause nexus of projection, but also as a mood Adjunct, just as probably does.  The reason for regarding this as a metaphorical variant is that the proposition is not, in fact, ‘I think”; the proposition is ‘it is so’.  This is shown clearly by the tag… .  It is the fact that a mental clause is a modal clause and serves as mood Adjunct that explains the tag… the Mood tag picks up the Mood element of the modalised proposition… .