Thomas Jefferson (2011). “The Papers of Thomas Jefferson, Retirement Series, Volume 7: 28 November 1813 to 30 September 1814”, p.648, Princeton University Press
Albert Einstein's speech made in honor of Thomas Mann, when Mann was given the Einstein Prize given by the Jewish Forum (January 1939), as quoted in Abraham Pais "Einstein Lived Here" (p. 214), 1994.
"A Dictionary of Thoughts: Being a Cyclopedia of Laconic Quotations from the Best Authors of the World, Both Ancient and Modern" by Tryon Edwards, New York, Cassell publishing company, (p. 517), 1891.
Stephen R. Covey (1994). “Daily Reflections for Highly Effective People: Living THE SEVEN HABITS OF HIGHLY SUCCESSFUL PEOPLE Every Day”, p.31, Simon and Schuster