Benjamin Franklin's letter to David Hartley (December 4, 1789) as quoted in "The Writings of Benjamin Franklin," Volume 10 (p. 72) edited by Albert H. Smyth, 1907.
Theodore Roosevelt (2015). “Theodore Roosevelt on Bravery: Lessons from the Most Courageous Leader of the Twentieth Century”, p.37, Skyhorse Publishing, Inc.
Chogyam Trungpa (2010). “The Collected Works of Chogyam Trungpa: Volume One: Born in Tibet; Meditation in Action; Mudra; Selected Writings”, p.309, Shambhala Publications
Alexander Hamilton, James Madison, John Jay (2016). “The Federalist Papers and the Constitution of the United States: The Principles of the American Government”, p.184, Skyhorse Publishing, Inc.