Mark Twain (2014). “Mark Twain on Common Sense: Timeless Advice and Words of Wisdom from America's Most-Revered Humorist”, p.31, Skyhorse Publishing, Inc.
Robert Collier (2013). “The Secret of the Ages: The Secret Edition - Open Your Heart to the Real Power and Magic of Living Faith and Let the Heaven Be in You, Go Deep Inside Yourself and Back, Feel the Crazy and Divine Love and Live for Your Dreams”, p.13, Lulu Press, Inc