Full Idea
In cases where the movement of the limbs that are the instruments of action has its origin in the agent himself, it is in his power either to act or not, and therefore such actions are voluntary.
Gist of Idea
An action is voluntary if the limb movements originate in the agent
Source
Aristotle (Nicomachean Ethics [c.334 BCE], 1110a15)
Book Reference
Aristotle: 'Ethics (Nicomachean)', ed/tr. ThomsonJ A K/TredennickH [Penguin 1976], p.111
A Reaction
He asserts this to show that an action is voluntary even under extreme compulsion or pressure. This seems right, and moves the focus to the concept of an 'excuse', which covers forgivable voluntary actions.