"Best You Ever: 365 Ways to be Richer, Happier, Thinner, Smarter, Younger, Sexier, and More Relaxed - Each and Every Day". Book by Rebecca Swanner (p. 324), October 18, 2010.
Albert Einstein (2015). “Bite-Size Einstein: Quotations on Just About Everything from the Greatest Mind of the Twentieth Century”, p.15, St. Martin's Press