Halliday & Matthiessen (1999: 578-9):
In the grammar, the 'idea' is a separate clause that combines with the 'sensing' clause in a clause complex, through the relation of projection; see Figure 14-4 for a structural interpretation.
In our analysis (unlike that of the mainstream grammatical tradition), the projected clause is not a constituent part of the mental or verbal clause by which it is projected. There are numerous reasons for this; some of them are grammatical — for example,
- it cannot be the focus of theme-predication [we do not say: it is that they're absolutely horrible that I think];
- it cannot be the Subject of a passive mental clause [we do not say: they're absolutely horrible is thought by me];
- it is presumed by the substitute so, which is also used to presume conditional clauses in clause complexes: I think they're absolutely horrible and my husband thinks so too].
But these, in turn, reflect the semantic nature of projection: this is a relationship between two figures, not a device whereby one becomes a participant inside another. We can thus show the difference between these and 'fact' clauses, those where the idea clause is a projection but it is not the accompanying mental clause that is doing the projecting; such readymade projections do function as constituents. An example here is I don't care whether they are devoted or not; compare it's not whether they are devoted or not that I care about.