Mark Twain, John Sutton Tuckey (1980). “The Devil's Race-track: Mark Twain's Great Dark Writings : the Best from Which was the Dream? and Fables of Man”, p.337, Univ of California Press
Letter to Mme. d'Épinal, Ferney on December 62, 1760. "Oeuvres Complètes de Voltaire: Correspondance". Book published by Garnier frères, Paris, Volume IX, letter #4390, p.124, 1881.