Halliday & Matthiessen (1999: 173):
The difference in the degree of involvement is also reflected in the extent to which an element is available for a critical role in the interpersonal metafunction. Prototypically, as we have noted, participants can be assigned the status of Subject, being made to carry the burden of the argument, whereas circumstances cannot. This distinction is however being obscured in Modern English, where although the prepositional phrase as a whole cannot function as Subject, the nominal group inside a prepositional phrase often can; e.g. the grass shouldn't be walked on.
Blogger Comments:
The obvious exceptions are circumstantial relational clauses, which are metaphorical.
