Turing's 1936 paper on computable numbers hit that rare bull's eye where philosophy and discovery overlap. The mathematician Alonzo Church at Princeton came to the same findings just months before Turing. But unlike Church, who used the standard abstractions of pure mathematics in his argument, Turing wrote of machines, algorithms, ink, paper tape, and computation. (Before Turing, a "computer" referred not to a machine, but to a human being who calculated with paper and pencil.)
This unique blend of philosophical speculation, eccentricity, and logical rigor was Turing's signature across his astonishing career. Later on, after the war, after he had snapped the German Engima cipher (while wearing pajamas), he took it upon himself to defend thinking machines. We would know machines could think, he claimed, when we couldn't tell if we were talking to a computer or a man, provided a wall or a door prevented us from seeing which was which.