Albert Einstein (2015). “Bite-Size Einstein: Quotations on Just About Everything from the Greatest Mind of the Twentieth Century”, p.32, St. Martin's Press
Marcus Tullius Cicero, Cyrus R. Edmonds (1863). “Three Books of Offices; Or, Moral Duties: Also His Cato Major, an Essay on Old Age; Laelius, an Essay on Friendship; Paradoxes; Scipio's Dream; and Letter to Quintus on the Duties of a Magistrate. Literally Translated, with Notes, Designed to Exhibit a Comparative View of the Opinions of Cicero, and Those of Modern Moralists and Ethical Philosophers”, p.245