Halliday & Matthiessen (2004: 286-8):
… while … there is clear evidence in the grammar for distinguishing one process type from another, there is also clear evidence for saying that, in a more abstract sense, every process is structured in the same way, on the basis of just one variable. This variable relates to the source of the process: what it is that brought it about. The question at issue is: is the process brought about from within, or from outside? … the variable is not one of extension but one of causation. Some participant is engaged in a process; is the process brought about by that participant, or by some other entity?