Thomas Jefferson, J. Jefferson Looney (2004). “The Papers of Thomas Jefferson, Retirement Series, Volume 9: 1 September 1815 to 30 April 1816”, p.263, Princeton University Press
Brian Tracy (2004). “Focal Point: A Proven System to Simplify Your Life, Double Your Productivity, and Achieve All Your Goals”, p.20, AMACOM Div American Mgmt Assn
David Starr Jordan (1983). “The strength of being clean: the physiological laws governing the pursuit of happiness ; The philosophy of hope : understanding the process by which hope displaces despair : an owners manual to the human soul”