Halliday & Matthiessen (2014: 243-4):
Material clauses do not necessarily represent concrete, physical events; they may represent abstract doings and happenings … But as the process becomes more abstract, so the distinction between Actor and Goal becomes harder to draw. … Even with concrete processes, however, we have to recognise that there are some where the Actor is involuntary, and thus in some respects like a Goal … With more abstract processes, we often find ‘operative’ and ‘receptive’ forms side by side with little difference between them … There is still some difference: if the ‘receptive’ form is used, we can probe for an explicit Actor — we can ask who by?, whereas with the ‘operative’ form we cannot. And this is what justifies us in still giving a different functional status to the participant in the two cases.