Halliday (2008: 35-6):
Then think of agency: how it is that processes are brought about. Either I do something, which may or may not impact on some other entity (I as Actor; plus or minus a Goal); or I do something, which may or may not be caused by some other entity (I as Medium; plus or minus an Agent). Whichever way we choose to model these, whether as extension (“transitive”) or as causation (“ergative”), we cannot know all about how processes with two participants take place.