Halliday & Matthiessen (1999: 193):
The general picture we are suggesting, then, is that it is the category of thing that the grammar captures to the greatest measure the complexity of the elemental phenomena of human experience. Put together with the different types of figure, which construe the complexity of goings-on upon the broad foundational categories of doing, sensing, saying and being, the different types of participant we have sketched in here foreground the dual nature of experience as both material and semiotic — a world that is constituted out of the interaction between entities and meanings. On each of these dimensions there is a progression from things that are most like to things that are least like ourselves.