Full Idea
Saying that human actions depend on the will is a mere phrase without any idea to correspond to. What the will is, and how it moves the body, no one knows.
Gist of Idea
Claiming that actions depend on the will is meaningless; no one knows what the will is
Source
Baruch de Spinoza (The Ethics [1675], II Pr 35)
Book Reference
Spinoza,Benedict de: 'Ethics, Improvement of Understanding, Letters', ed/tr. Elwes,R [Dover 1955], p.109
A Reaction
As so often, the rationalist Spinoza agrees with many empiricists about this one. If there is no such thing as the will, then there isn't much prospect of it being free, thought one might talk about 'freedom of thought' instead.