Halliday & Matthiessen (1999: 218):
Type (2), those circumstantial elements that are realised by prepositional phrases, are rather more complex, since they include another element — a participant — in their makeup ("macro circumstances"): e.g. the table, my knowledge, peace and quiet in right under the table, without my knowledge, for the sake of peace end quiet.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 Process in the main figure but is nevertheless related systematically to the spectrum of process types — mainly, though not exclusively, to processes of being.