From Denis Diderot’s Pensees Philosophiques (no. 32), though I found the quote through Baudelaire’s La Fanfarlo, where he presents it as the something like the key to his main character’s behavior (see further below):
Incredulity is sometimes the vice of a fool, and credulity the failing of a man of intelligence. The man of intelligence sees far into the immensity of possibilities; the fool sees hardly anything as possible except what already exists. It is this perhaps which makes the one a coward, the other rash.
L’incrédulité est quelquefois le vice d’un sot, et la crédulité le défaut d’un homme d’esprit. L’homme d’esprit voit loin dans l’immensité des possibles ; le sot ne voit guère de possible que ce qui est. C’est là peut-être ce qui rend l’un pusillanime, et l’autre téméraire.
Baudelaire’s further commentary:
This pensee of Diderot explains as well all the blunders Samuel committed in his life, blunders that a fool would not have committed. This portion of the public that is essentially cowardly will hardly understand the character of Samuel, who was essentially credulous and rich in imagination, to the point that, as poet, he believed in his public – as man, in his own passions.
La pensée de Diderot …. explique aussi toutes les bévues que Samuel a commises dans sa vie, bévues qu’un sot n’eût pas commises. Cette portion du public qui est essentiellement pusillanime ne comprendra guère le personnage de Samuel, qui était essentiellement crédule et imaginatif, au point qu’il croyait, comme poëte, à son public, — comme homme, à ses propres passions.
Extracts 