Halliday & Matthiessen (2014: 539):
While there is no participant doing the projecting – no Sayer or Senser – a fact may be projected impersonally, either by a relational process (‘it is the case that ...’) or by an impersonal mental or verbal process, as in
[i] relational
it is/may be/is not (the case) that ...
it happens (to be the case) that ...
it has been shown/can be proved (to be the case) that ...
it happened/came about that ...
[ii] mental: impersonal
it seems/appears/is thought (to be the case) that ...
[iii] verbal: impersonal
it is said/rumoured (to be the case) that ...
Here the it is not a participant in the projecting process but is simply a Subject placeholder; hence the fact clause can occupy its position at the front: that Caesar was ambitious is certainly the case/is widely held/is generally believed, etc. By contrast we do not normally say that Caesar was ambitious was thought/said by Brutus – at least not in a reporting context, only in the special sense of ‘these lines were spoken by ...’; and this is because, as we have seen, where there is a personal projecting process, mental or verbal, the clause that is projected by it is not embedded but hypotactic.
Blogger Comments:
Lest this wording be misunderstood, the impersonal projection of a fact occurs within a single clause, since the fact is embedded; the projection is not a relation between ranking clauses in a clause complex.