Albert Einstein (2015). “Bite-Size Einstein: Quotations on Just About Everything from the Greatest Mind of the Twentieth Century”, p.32, St. Martin's Press

Remark made during a week at Princeton beginning 9 May 1921, later carved above the fireplace of the Common Room of Fine Hall (the Mathematical Institute), Princeton University - in R. W. Clark Einstein (1973) ch. 14

"THE SIMPLE AND THE COMPLEX Part I: THE QUANTUM AND THE QUASI-CLASSICAL with MURRAY GELL-MANN, Ph.D". Interview with Jeffrey Mishlove, www.intuitionnetwork.org.