Full Idea
I say that a thing is free, which exists and acts solely by the necessity of its own nature.
Gist of Idea
A thing is free if it acts only by the necessity of its own nature
Source
Baruch de Spinoza (Letter to G.H. Schaller [1674], 1674.10)
Book Reference
Spinoza,Benedict de: 'Ethics, Improvement of Understanding, Letters', ed/tr. Elwes,R [Dover 1955], p.390
A Reaction
Of course, this isn't 'freedom' at all, but it seems to exactly right as an account of so-called freedom. In the case of a human being the 'necessity of our own nature' is character, and virtue and vice are the expressions of the necessities of character.