Halliday & Matthiessen (1999: 512):
The basic component of all experience is change: when something changes from one state to another, it projects itself on to our consciousness. This may be something in the external environment … The grammar construes this experience of change in the form of a process configuration: the fundamental element of grammar is the clause, and the clause presents the parameters within which processes may unfold.
The grammar does this by deconstructing the process into component parts. Typically … these are of three kinds: first the process itself, secondly certain phenomena construed as participants in the process, and thirdly, other phenomena that are associated with the process circumstantially.