Full Idea
If the action is not caused by some other event, and it is not causeless, this leaves the possibility that it is caused by something else instead, and this something can only be the agent, the man.
Gist of Idea
If actions are not caused by other events, and are not causeless, they must be caused by the person
Source
Roderick Chisholm (Human Freedom and the Self [1964], p.28)
Book Reference
'Free Will', ed/tr. Watson,Gary [OUP 1982], p.28