Halliday & Matthiessen (1999: 577):
We have shown how the system of the ideation base construes consciousness: as conscious processing by a conscious being. Conscious processing can create a higher–order world of ideas (or, as we would say, meanings), comparable in certain respects to Popper’s World 3; this defines the essential distinction between projection and expansion as ways of relating one figure to another. Conscious processes themselves appear as the central figures in the construal of experience, and they are pivotal in differentiating among various types of participant. … Conscious processes are of two kinds: sensing, and saying.