Halliday & Matthiessen (1999: 222):
These two motifs together have made it possible for human beings to transform experience into meaning, taking the experience of meaning itself — the "inner" processes of consciousness — as the central figures, and those with the ability to mean — prototypically humans themselves — as the central participants. These then serve as the point of reference for construing "outer" experience, the complementary experiences of the processes of doing and of being.