His luminous intelligence and the brilliance of his team (some of them recruited in a competition to see who could finish a newspaper crossword puzzle the fastest) allowed the British to begin secretly to decrypt signals from the Enigma and Lorenz machines, used for communication between German war headquarters and deployed U-boats, and land an airborne forces.
"I won't say that what Turing did made us win the war," his statistical clerk, Jack Good, told me later, as I was researching a book that would be the first history of artificial intelligence, called Machines Who Think, "but I daresay we might have lost it without him."
I interviewed Good in the mid-1970s, when the Official Secrets Act still silenced many. Only decades later were Turing's associates, and then, historians of computing, mathematics, and cryptography, able to reveal in detail how crucial Turing's contribution had been to the war effort, to the ultimate Allied victory, and to hungry babies like me, born into what then looked like a losing effort.
A man who had saved his country was hounded to death by it.
My first acquaintance with Turing was related to my research in the history of AI. His work in the field was groundbreaking -- and emerged soon after he was able to lay down his wartime responsibilities.
During a sabbatical at his old school, King's College, Cambridge, he wrote a paper in September 1948 called "Intelligent Machinery." Unpublished for almost two decades, its intellectual penetration is startling even today. In it he proposed the possibility of machine intelligence, beginning with arguments against the idea: that humans will not admit the possibility of rivals in intellectual power; that such a machine, if possible, would be irreverent; that machinery and humans are vastly different; that Gödel's Theorem shows that machines are inherently incapable of solving problems which humans can overcome; and that, anyway, machine intelligence is no more than a reflection of the intelligence of its creator. And then he refuted each one -- brilliantly.
More famously, in October 1950 he published "Computing Machinery and Intelligence," in which he proposed what came to be known as the Turing Test, consisting of an interrogation between a human judge and hidden respondents -- the human judge to decide whether the respondents were human or machine. The paper caused a scientific sensation, and eminent critics rose hotly to defend human intelligence. Turing responded in written and oral debate with great coolness, élan, and humor.
By now Turing had conducted some experiments that informed his arguments. He'd designed one of the first ever chess-playing programs (for a machine that didn't yet exist) and he was growing interested in the operations of the human brain, which might, he hoped, give him deeper insights into how to construct a thinking machine.