Doubt is a proof of modesty; it has seldom harmed the advance of the sciences. I could not say as much for incredulity. Whoever, apart from pure mathematics, pronounces the word “impossible” is wanting in prudence.
Francois Arago, Éloge, in Oeuvres complètes, vol. 2 (Paris: Gide & J. Baudry, 1854), p. 313.