Quoted in Project Cyclops: A Design Study of a System for Detecting Extraterrestrial Intelligent Life, rev. ed., ed. B. M. Oliver and J. Billingham (1973). An earlier version by A. R. Burn appeared in a book review by Burn in The Classical Review, June 1969: "absence of evidence is not identical with evidence of absence."
Albert Einstein (2015). “Bite-Size Einstein: Quotations on Just About Everything from the Greatest Mind of the Twentieth Century”, p.40, St. Martin's Press
Charles Darwin (1861). “On the Origin of Species by Means of Natural Selection; Or, The Preservation of Favoured Races in the Struggle for Life”, p.167