He who has imagination without learning has wings and no feet.

[Celui qui a de l’imagination sans érudition, a des ailes et n’a pas de pieds.]

Joseph Joubert (1754-1824) French moralist, philosopher, essayist, poet

Pensées [Thoughts], ch. 4 “De la Nature des Esprits [On the Nature of Minds],” ¶ 39 (1850 ed.) [tr. Attwell (1896), ¶ 53]
    (Source)

(Source (French)). Alternate translations:

The man of imagination without learning has wings and no feet.
[tr. Lyttelton (1899), ch. 3, ¶ 16]

The man of imagination who is unlearned has wings and no feet.
[tr. Collins (1928), ch. 4]