Halliday & Matthiessen (1999: 194):
… the grammar, in its role 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 (i.e. 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. By reference to the grammar of the clause on the one hand, and of the nominal group on the other, certain broad categories are set up such that some things will fall squarely into one category, while others will lie on the borderline, showing certain features of one category and certain features of another, or finding themselves equally at home in both.