Full Idea
There must be some event A, presumably some cerebral event, which is not caused by any other event, but by the agent.
Gist of Idea
There has to be a brain event which is not caused by another event, but by the agent
Source
Roderick Chisholm (Freedom and Action [1966], p.20), quoted by Rowland Stout - Action 4 'Agent'
Book Reference
Stout,Rowland: 'Action' [Acumen 2005], p.65
A Reaction
I'm afraid this thought strikes me as quaintly ridiculous. What kind of metaphysics can allow causation outside the natural nexus, yet occuring within the physical brain? This is a relic of religious dualism. Let it go.