Mark Twain (2017). “The Complete Works of Mark Twain (Illustrated Edition): Novels, Short Stories, Memoir, Travel Books, Letters, Biography, Articles & Speeches: The Adventures of Tom Sawyer & Huckleberry Finn, Life on the Mississippi, Yankee in King Arthur's Court…”, p.8291, e-artnow
Attributed to Aristotle in Diogenes Laertius "The Lives and Opinions of Eminent Philosophers" translated by C. D. Yonge and Henry G. Bohn (p. 188), 1853.