Truth is mighty and will prevail. There is nothing wrong with this, except that it ain’t so.

Mark Twain (1835-1910) American writer [pseud. of Samuel Clemens]

Mark Twain’s Notebook (1935) [ed. Albert Bigelow Paine]
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With an entry for 4 Jul 1893. The core phrase, from the Latin "Magna est veritas et prævalebit," was first formulated in English by Thomas Brooks. An earlier variant can be found in Cicero, Pro Caelio Rufo (56 BC): "How great is the power of truth" [O magna vis veritas].