Halliday & Matthiessen (1999: 194):
… the grammar, in its rôle as a theory of human experience, categorises those phenomena that it construes as participants by locating them in a spectrum based on a scale of distance from the human — at one end humans themselves, and things most similar to (ie categorisable as) humans, at the other end things that are farthest away from being human: concrete substances in the material world and abstract “substances” in the semiotic world.