Halliday & Matthiessen (1999: 153):
What we find, if we try to take in the picture as a whole, is a kind of focussed model in which the essentially human processes of consciousness [sensing], and the prototypically human processes of symbolic action [saying], constitute the experiential centre; while the two other types of figure, that of doing on the one hand and of being on the other, lie on opposite sides of this centre: the one (doing) lying towards the pole of the concrete, with the experience construed as 'this impacts on that', the other (being) lying towards the pole of the abstract, with the experience construed as 'this is related to that'.