Halliday & Matthiessen (2004: 472):
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: impersonalit seems/appears/is thought (to be the case) that …[iii] verbal: impersonalit 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 Cæsar was ambitious is certainly the case/is widely held/is generally believed, etc.