Halliday & Matthiessen (2014: 311):
There is thus a continuity between the categories of participant and circumstance; and the same continuity can be seen in the forms by which the two are realised. The distinction between participant and circumstance is probably relevant in all languages; but in some it is drawn relatively sharply, while in others it is shaded and blurred. We shall see … that it has become blurred in English, and for an interesting reason: it has been superseded by something else.