In his landmark 1950 paper “Computing Machinery and Intelligence,” the mathematician, philosopher and code breaker Alan Turing proposed a method for answering the question “Can machines think?”: an “imitation game” in which an “interrogator,” C, interviews two players, A and B, via teleprinter, then decides on the basis of the exchange which is human and which is a computer.

Turing’s radical premise was that the question “Can a machine win the imitation game?” could replace the question “Can machines think?” — an upsetting idea at the time, as the neurosurgeon Sir Geoffrey Jefferson asserted in 1949: “Not until a machine can write a sonnet or compose a concerto because of thoughts and emotions felt, and not by the chance fall of symbols, could we agree that machine equals brain — that is, not only write it but know that it had written it.” Turing demurred: if the only way to be certain that a machine is thinking “is to be the machine and to feel oneself thinking,” wouldn’t it follow that “the only way to know that a man thinks is to be that particular man”? Nor was the imitation game, for Turing, a mere thought experiment. On the contrary, he predicted that in 50 years, “it will be possible to program computers . . . to make them play the imitation game so well that an average interrogator will not have more than 70 percent chance of making the right identification after five minutes of questioning.”

Well, he was almost right, as Brian Christian explains in “The Most Human Human,” his illuminating book about the Turing test. In 2008, a computer program called Elbot came just one vote shy of breaking Turing’s 30 percent silicon ceiling. The occasion was the annual Loebner Prize Competition, at which programs called “chatterbots” or “chatbots” face off against human “confederates” in scrupulous enactments of the imitation game. The winning chatbot is awarded the title “Most Human Computer,” while the confederate who elicits “the greatest number of votes and greatest confidence from the judges” is awarded the title “Most Human Human.”

It was this title that Christian — a poet with degrees in computer science and philosophy — set out, in 2009, to win. And he was not about to go “head-to-head (head-to-motherboard?) against the top A.I. programs,” he writes, without first getting, as it were, in peak condition. After all, for Elbot to have fooled the judges almost 30 percent of the time into believing that it was human, its rivals had to have failed almost 30 percent of the time to persuade the judges that they were human. To earn the “Most Human Human” title, Christian realized, he would have to figure out not just why Elbot won, but why humanity lost. . . .