Alan Turing
Alan Turing (1912–1954) was a British mathematician and logician who, in a single decade, defined the mathematical limits of computation, helped break German naval Enigma traffic at Bletchley Park, drew up one of the first complete designs for a stored-program electronic computer, and posed the question of machine intelligence in the form that still organizes the field. He is the central figure of this glossary series, but the person is a different subject from his artifacts: the machine model, the universal machine, the 1950 paper and the imitation game each have their own entries, and this one is about the life, the career and the legacy.
The career begins with a pure-mathematics result in 1936. Born in London on 23 June 1912, Turing became a fellow of King's College, Cambridge, in 1935. His paper "On Computable Numbers, with an Application to the Entscheidungsproblem," read to the London Mathematical Society in November 1936 and printed in its Proceedings, answered a question David Hilbert and Wilhelm Ackermann had posed in 1928 about whether a mechanical procedure could decide the truth of any logical statement. Turing's answer was no, and to prove it he had to say precisely what a mechanical procedure is. Alonzo Church had reached the same negative verdict a little earlier by a different route, and Turing went to Princeton to work under Church, taking his doctorate there in 1938.
The war made him a practical engineer of codebreaking. From September 1939 Turing worked at the Government Code and Cypher School at Bletchley Park, where he led the attack on the German naval Enigma in Hut 8. He contributed to statistical methods such as Banburismus and to the design of the electromechanical bombe, a machine that tested possible Enigma settings against a guessed fragment of plaintext; the bombe descended from earlier Polish work, and Gordon Welchman's diagonal board made it far more efficient. Codebreaking at Bletchley was a team effort involving thousands of people, and the accurate claim is that Turing was one of its most influential figures, not that he single-handedly won a war. He received an OBE in 1946, but wartime secrecy kept the work out of public view for decades.
After the war he moved from theory to building and programming real machines. In 1945 he joined the National Physical Laboratory and wrote the design for the Automatic Computing Engine, or ACE, a fast stored-program machine whose ambition outran the laboratory's engineering capacity; a reduced Pilot ACE first ran in 1950. In 1948 he moved to the University of Manchester, where the Manchester Small-Scale Experimental Machine had run the first stored program in June 1948, and he worked on programming the Manchester computers that followed. His 1950 paper "Computing Machinery and Intelligence" appeared in the philosophy journal Mind, and in 1951 he was elected a Fellow of the Royal Society. His late work turned to mathematical biology, most notably a 1952 paper on how chemical reaction and diffusion could generate biological patterns.
His prosecution and death are part of the history, not a footnote to it. In 1952 Turing was convicted of gross indecency under the law then criminalizing homosexual acts between men, and accepted a hormone treatment regimen rather than prison. He died on 7 June 1954 at his home in Wilmslow, and an inquest recorded cyanide poisoning as a suicide. The British government issued a public apology in 2009, and Queen Elizabeth II granted a royal pardon in December 2013; legislation sometimes called the Turing law later extended pardons to others convicted under the same statutes. A portrait of him appeared on the Bank of England fifty-pound note issued in 2021.
The honors that carry his name keep the ideas in circulation. The Association for Computing Machinery established the A. M. Turing Award in 1966, and it is usually described as computing's equivalent of a Nobel prize; its first recipient was Alan Perlis. In day-to-day usage, "Turing complete" labels any language or system able to compute everything a Turing machine can, "Turing test" names the imitation game's descendants, and undecidability results trace directly to his 1936 argument. In current AI, the line from his 1950 speculation about learning machines to machine-learning systems that are trained, not hand-programmed, is easy to draw, though he would not have recognized today's methods.
Common misconceptions credit him with too much or too little. He did not invent the computer alone: Babbage, Konrad Zuse, the ENIAC and EDVAC groups, von Neumann and many others built the field, and historians still debate how directly his 1936 paper shaped the first stored-program designs. He also did not break Enigma alone, nor did he propose the test as a literal certification of intelligence. Equally wrong is treating him as a mere codebreaker whose theory was incidental: the 1936 result is the foundation under computer science itself, and the later biography is not separable from its social context.
Read Alan Turing as the person who made "mechanical procedure" a precise mathematical concept, then spent the rest of a short life testing how far that concept could be pushed into engineering, biology and the philosophy of mind.