Halliday & Matthiessen (1999: 518-9):
The line between direct and indirect participants is a fuzzy one, and sometimes what seems to be the same rôle in the process can be construed in either way … Here the grammar is in fact using the structural resource of plus or minus preposition to construe a different kind of contrast, having to do with status in the message. But the distinction is significant because, as we saw above, such “circumstantial” elements tend also to function as qualifications not of the process but of some entity that is itself a participant … And since the prepositional phrase has a nominal group inside it, this opens up the possibility of further expansion …. Thus incorporating the circumstantial element into the representation of a participant does not merely add one feature to the specification; it allows more or less indefinite scope, particularly in combination with the incorporation of an entire process. (We have already pointed out that the prepositional phrase is in fact a miniaturised clause; so the two really constitute a single resource, that of using a process to specify a particular class of entity.) This potential was crucial to the development of science and mathematics ….