Albert Einstein, Stephen W. Hawking (2007). “A Stubbornly Persistent Illusion: The Essential Scientific Works of Albert Einstein”, p.346, Running Press
" A Dictionary of Thoughts: Being a Cyclopedia of Laconic Quotations from the Best Authors of the World, Both Ancient and Modern" by Tryon Edwards, New York, Cassell publishing company, (p. 134), 1891.