The Secret Life of I. J. Good

I. J. (“Jack”) Good was a leading Bayesian statistician for more than half a century after World War II, playing an important role in the postwar Bayesian revival. But his graduate training had been in pure mathematics rather than statistics (one of his doctoral advisors at Cambridge had been the famous G. H. Hardy). What was responsible for this metamorphosis from pure mathematician to applied and theoretical statistician? As Good himself only revealed in 1976, during the war he had initially served as an assistant to Alan Turing at Bletchley Park, working on the cryptanalysis of the German Naval Enigma, and it was from Turing that he acquired his life-long Bayesian philosophy. Declassified and other documents now permit us to understand in some detail how this came about, and indeed how many of the ideas Good explored and papers he wrote in the initial decades after the war, in fact, gave in sanitized form, results that had their origins in his wartime work. Drawing on these sources, this paper discusses the daily and very real use of Bayesian methods Turing and Good employed, and how this was gradually revealed by Good over the course of his life (including his return to classified work in the 1950s).