Halliday & Matthiessen (1999: 218):
Those circumstantial elements that are realised by prepositional phrases are rather more complex, since they include another element — a participant — in their makeup. In such cases the element realised by the nominal group is still functioning as a participant in the process — but indirectly, being implicated only through the mediation of a preposition. That this is possible is because the preposition itself constitutes a subsidiary kind of ‘process’; one that does not function as a process in the main figure but is nevertheless related systematically to the spectrum of process types — mainly, though not exclusively, to processes of being.