What inconceivable madness! For it is not enough to call an opinion “foolishness” when it is utterly devoid of reason.

[O delirationem incredibilem! non enim omnis error stultitia dicenda est.]

Marcus Tullius Cicero (106-43 BC) Roman orator, statesman, philosopher

De Divinatione [On Divination], Book 2, ch. 43 (2.43) / sec. 90 (44 BC) [tr. Falconer (1923)]
    (Source)

(Source (Latin)). Alternate translations:

• "What an incredible insanity this is! for every error does not deserve the mere name of folly." [tr. Yonge (1853)]
• "We must not say that every mistake is a foolish one." This is an early and quite common English translation of the phrase (e.g.) and seems to reverse the meaning.