As you have it in your power, sir, to do some service to letters, I implore you not to clip the wings of our writers so closely, nor to turn into barn-door fowls those who, allowed a start, might become eagles; reasonable liberty permits the mind to soar — slavery makes it creep.

[Puisque vous êtes, monsieur, à portée de rendre service aux belles-lettres, ne rognez pas de si près les ailes à nos écrivains, et ne faites pas des volailles de basse-cour de ceux qui, en prenant l’essor, pourraient devenir des aigles: une liberté honnête élève l’esprit, et l’esclavage le fait ramper.]

Voltaire (1694-1778) French writer [pseud. of Francois-Marie Arouet]

Letter (1733-06-20) to a First Commissioner [Lettre á un premier commis] [tr. Tallentyre (1919)]
    (Source)

(Source (French); alternate).

On censorship. The last clause is most often quoted, and has spawned a variety of French and English variants, e.g.:

• [La servitude fait ramper l'esprit, et la liberté le fait voler.]
• Slavery makes the mind creep, and liberty makes it fly.