" 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.
Albert Einstein, Stephen W. Hawking (2007). “A Stubbornly Persistent Illusion: The Essential Scientific Works of Albert Einstein”, p.346, Running Press