Halliday & Matthiessen (2004: 286-8):
The arguments for the ergative interpretation are long and technical. But while, as we have seen, 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?
This is not the same thing as the intransitive/transitive distinction. There, as we saw, the variable is one of extension. The Actor is engaged in a process; does the process extend beyond the Actor, to some other entity, or not?