Halliday & Matthiessen (1999: 218):
Looking at them as oblique “cases”, from the point of view of their function in the larger figure, two points emerged: one, that participants and circumstances taken together formed a cline, rather than being separated by a clear boundary; the second, that some of the circumstantial elements could be ‘paired off’ with participants, being seen as a more oblique manifestation of a similar rôle.