Full Idea
Liberty, which is but a power, belongs only to agents, and cannot be an attribute or modification of the will, which is also but a power.
Gist of Idea
Liberty is a power of agents, so can't be an attribute of wills
Source
John Locke (Essay Conc Human Understanding (2nd Ed) [1694], 2.21.14)
Book Reference
Locke,John: 'Essay Concerning Human Understanding', ed/tr. Nidditch,P.H. [OUP 1979], p.240
A Reaction
He is suggesting the 'free will' is a category mistake, but why shouldn't a power have a power? Magnetism can be strong, or focused. He is ducking the question of what ultimately controls the will.