Full Idea
The determinism which worries the defender of free will is that if human action is subject to a universal law of causation, there will be for any action a set of sufficient conditions which can be traced back to factors outside the control of the agent.
Gist of Idea
Determinism threatens free will if actions can be causally traced to external factors
Source
Philippa Foot (Free Will as Involving Determinism [1957], p.63)
Book Reference
Foot,Philippa: 'Virtues and Vices' [Blackwell 1981], p.63
A Reaction
She draws on Russell for this, but neither of them mention whether the causation is physical. Free will seems to imply non-physical causation.