“Nothing is so firmly believed as that which we least know.”
... il n'est rien creu si fermement que ce qu'on sçait le moins, ...
Book I, Ch. 31
Essais (1595), Book I
Variant: Nothing is so firmly believed as what is least known.
Source: The Complete Essays
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Michel De Montaigne 264
(1533-1592) French-Occitan author, humanistic philosopher, … 1533–1592Related quotes

“They weren't lying. They firmly believed it all. Which doesn't change the facts.”
Source: The Last Wish

“WE ARE COMPELLED, OUR FAITH URGING us, to believe and to hold—and we do firmly believe and simply confess—that there is one holy Catholic and Apostolic Church, outside of which there is neither salvation nor remission of sins”
Unam sanctam ecclesiam catholicam et ipsam apostolicam urgente fide credere cogimur et tenere, nosque hanc frmiter credimus et simpliciter confitemur, extra quam nec salus est, nec remissio peccatorum,
Unam sanctam (1302)

“I believe that we should die with decency so that at least decency will survive.”
Markings (1964)

As quoted in The Truth in Words (2005) by Neal Zero

Address http://reto.cn/biblioteko/movado/zamenhof/paroladoj/uk2.htm to the Second World Congress of Esperanto, Geneve, Switzerland. 27 August 1906.