Albert Einstein (2015). “Bite-Size Einstein: Quotations on Just About Everything from the Greatest Mind of the Twentieth Century”, p.31, St. Martin's Press
Wallace D. Wattles (2015). “The Science of Being Great: Personal Self-Help Book of Wallace D. Wattles (Unabridged): From one of The New Thought pioneers, author of The Science of Getting Rich, The Science of Being Well, How to Get What You Want, Hellfire Harrison, How to Promote Yourself and A New Christ”, p.27, e-artnow