Misunderstanding Metaphors Of Modality

Martin & Rose (2007: 235):

One variation on this principle to watch out for involves certain mental process clauses in first or second person present tense (I think..., I suppose.. ., do you reckon..., don't you suppose, etc.). These are actually modalities involving grammatical metaphor (Halliday and Matthiessen 2004).

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This misunderstands metaphors of modality. As this concerns explicit subjective modality, it is only first person variants that function this way. Halliday & Matthiessen (2014: 687) explain, beginning with the example I don't believe that pudding ever will be cooked:

Here the cognitive mental clause I don’t believe is a metaphorical realisation of probability: the probability is realised by a mental clause as if it was 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; if we tag the clause I think it’s going to rain we get 

I think it’s going to rain, isn’t it?

not I think it’s going to rain, don’t I?. In other words the clause is a variant of it’s probably going to rain (isn’t it?) and not a first-person equivalent of John thinks it’s going to rain, which does represent the proposition ‘John thinks’ (tag doesn’t he?). Thus in

You know || what’s happening tomorrow at five o’clock, don’t you?

the ‘mental’ clause you know was able to be tagged because it does not stand for a modality. It is the fact that mental clause is a modal clause and serves as mood Adjunct that explains the tag. If it was just an ordinary mental clause in a clause nexus of projection, I don’t believe should be able to be tagged. But since it has a metaphorical status and serves as mood Adjunct, it cannot be tagged. Instead, the Moodtag picks up the Mood element of the modalised proposition: that pudding probably never will ... will it?