A process consists potentially of three components: (i) the process itself; (ii) participants in the process; (iii) circumstances associated with the process. These provide the frame of reference for interpreting our experience of what goes on. Imagine that we are in the open air and that there is movement overhead [...] something which we express as, say, 'birds are flying in the sky' [...] In this interpretation of what is going on, there is doing ['are flying'], a doer ['birds'], and a location ['in the sky'] where the doing takes place. This tripartite interpretation of processes is what lies behind the grammatical distinction of word classes into verbs, nouns, and the rest, a pattern that in some form or other is probably universal among human language.
- Halliday (1985), a pag.101-102