Halliday & Matthiessen (1999: 517):
At the same time, all processes are interpreted as having something in common, in that typically there is one participant that is inherently associated with the process — without which the process could not take place at all, like birds in birds are flying. That may be the only one, in which case that participant is held accountable (even if involuntarily!) and the process is said to stop there. Alternatively, another participant may be involved; either as an external agent, like children in children are flying kites, or as a goal, like letter in I’m writing a letter.