Alan Turing
British mathematician who invented a conceptual machine with his name (Turing machine), which is quite useful for characterizing the essence of computation. Computability by means of a Turing machine can be clearly defined, and it can replace our informal and intuitive notion of computation (Church-Turing Thesis); but as it turns out, it has a definite limitation. "Can there be a Turing machine which can decide, for any pair of a Turing machine and its input tape, whether it will stop after a finite amount of time?" (Halting Problem) Turing proved that there cannot be any such machine (thus the halting problem is one of the "undecidable problems"). This has a close relationship with Goedel's Incompleteness Theorem.
Turing worked, during the war, for manufacturing an actual computing machine: the Colossus for deciphering the German cryptography produced by the machine "Enigma". And after the war, Turing published an influential paper "Computing Machinery and Intelligence" (Mind, 1950) in which he proposed the "Turing Test" for judging whether or not something is "intelligent". This paper soon became a "classic" both in the field of artificial intelligence and philosophy of mind.
Turing was arrested in 1952 on account of a homosexual act, "an act of gross indecency", and he was put on probation with the condition that he receive medical treatment (which meant a drug or hormone treatment for reducing sexual urge). The probation period ended in April 1953; but about a year later, he (unexpectedly) killed himself, June 7, 1954.
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