Halliday & Matthiessen (1999: 513, 515):
The significant step that took place in human grammars in this context was, obviously, the evolution of common nouns — or rather, of common words, since verbs are also "common" in this sense: that is, words denoting classes rather than individuals. It is usually assumed that these evolved out of "proper" words, prototypically the names of individual persons; the ontogenetic evidence suggests that this is one source but not the only one, another source being rather in the interpersonal function. Be that as it may, construing processes in this way clearly depends on generalising whole classes of phenomena; the grammar sets up classes of process, of participant and of circumstance. There are various ways of doing this; one that is familiar in many languages is by means of a taxonomy of different kinds of word, as in Figure 13-1. The classes of word may be distinguished by their internal form, or by the way they are able to enter into larger constructions (or both).
