July 8, 2026

Alan Turing: The 1936 Idea at the Heart of Every Computer, and the 1952 Conviction That Broke Its Author

He invented the concept behind every computer, broke the cipher that helped win the Atlantic, and asked the question that still organizes AI. The state he saved then chemically castrated him for loving another man.

In the Wilmslow police station in January 1952, Alan Turing did the thing that would destroy him: he told the truth. His house had been burgled. He had a shrewd idea who was responsible, a nineteen-year-old named Arnold Murray he had met on Oxford Road and brought home. When detectives pressed him on how he knew Murray, Turing, caught in a lie about a fingerprinted glass, stopped dissembling and gave them a complete account of the affair. It was, an officer told him, a "lovely statement." Each of three nights counted as two offences under the 1885 law. Six charges.

As the implication landed, Turing put a question to the policeman that, in its calm assumption that the world was about to catch up with him, sounds like almost no one else: "What is going to happen about all this? Isn't there a Royal Commission sitting to legalise it?" It wasn't. Homosexuality would not be decriminalised in Britain for another fifteen years. The man reporting a burglary as though the law were a temporary inconvenience had, twelve years earlier, broken the cipher Germany used to command its U-boats, and sixteen years before that, dreamt up the idea of the machine you are reading this sentence on. He was simply early.

His life is one question asked three ways: as a grieving teenager, does the mind survive the body; as a logician, what can a machine compute; in print at thirty-eight, can a machine think. He spent twenty years turning each vague question into a precise one, and the precision made him dangerous, to mathematics, to the German navy, and finally to himself.

The machine that could be any machine

The question Turing is famous for, "Can machines think?", opens his 1950 paper Computing Machinery and Intelligence, in the journal Mind. He immediately calls it a bad question: defining the words by ordinary usage would, he wrote, reduce the whole thing to "a statistical survey such as a Gallup poll," which he called "absurd." So he replaced it with a game: a man and a woman sit in one room, an interrogator in another; the interrogator must decide which is which. Swap the man for a machine; if the interrogator fails as often as before, the machine has passed. He called it the imitation game. The move from "can machines think?" to "can a machine win this game?" is the whole of his method: take a foggy question, find the operational test hidden inside it, and run it.

Then he made a forecast so specific it is now read for sport. "I believe that in about fifty years' time it will be possible to programme computers, with a storage capacity of about 10 to the power 9, to make them play the imitation game so well that an average interrogator will not have more than 70 per cent chance of making the right identification after five minutes of questioning." He added that the original question was "too meaningless to deserve discussion," but that "at the end of the century the use of words and general educated opinion will have altered so much that one will be able to speak of machines thinking without expecting to be contradicted." The paper is seventy-five years old and still the frame every debate about large language models falls back into, usually without knowing it.

The reason a 1950 paper could speak of programming computers at all was a quieter idea he had published fourteen years earlier, in 1936, while a twenty-three-year-old fellow at King's College, Cambridge. "On Computable Numbers, with an Application to the Entscheidungsproblem" set out to solve a problem in logic, whether a definite mechanical procedure exists for deciding if any proposition is provable. To answer it, Turing had to define "mechanical," inventing an abstract device, soon called the Turing machine, a head reading and writing symbols on an infinite tape by a table of rules. Then came the move that turned a logic paper into the blueprint of a century. Section 6: "It is possible to invent a single machine which can be used to compute any computable sequence." One fixed machine, given the description of any other on its tape, would mimic that other exactly. Turing later put it in plainer words: "It can be shown that a single special machine of that type can be made to do the work of all. The special machine may be called the universal machine."

That is the stored-program concept, the principle that complexity lives in the instructions on the tape, not in the hardware, which is why your phone runs a calculator on the same silicon. He had, in effect, invented the computer to prove a theorem about what computers could not do: no general procedure exists for deciding provability, because some things are not computable. He set a ceiling over his invention the day he built it.

The boy who theorised the spirit

Before there was a universal machine, there was a dead friend. At Sherborne School in 1928, Turing found the one person who could keep up with him, Christopher Morcom, a year older, a mathematician and astronomer who, as his housemaster recorded Turing saying, "was never quite satisfied" with a result but "considered every detail of it as a separate problem." In December 1929 they sat the Trinity College, Cambridge scholarship exams together. Morcom won; Turing did not. They were to be separated for a year.

Then, in February 1930, after a concert of part-songs at the school, Morcom fell ill and, after two operations in London, died at noon on 13 February. He was eighteen and a half; Turing was eighteen.

On the day of the funeral service, Turing wrote to his mother, Sara: "I feel sure that I shall meet Morcom again somewhere & that there will be some work for us to do together, as I believed there was for us to do here. Now that I am left to do it alone I must not let him down but put as much energy into it, if not as much interest, as if he were still here." Visiting the family home, the Clock House, in about April 1932, he wrote an essay called "Nature of Spirit", now held in the Turing Archive at King's College, Cambridge, in which the boy who would mechanise thought tried to save the soul. Modern physics, he argued, had broken determinism at the scale of atoms, so "the theory which held that as eclipses etc. were predestined so were all our actions, breaks down too." The body, he wrote, "by reason of being a living body can 'attract' & hold on to a 'spirit' whilst the body is alive & awake," and at death "the 'mechanism' of the body, holding the spirit is gone & the spirit finds a new body sooner or later perhaps immediately."

Read alongside the 1936 and 1950 papers, a single investigation appears across twenty years: can the "mechanism" of the body hold a mind, and could a different mechanism, of tape and valves, hold one too? Morcom's death is usually told as a tragedy that scarred him; it is also the first draft of his life's work.

The state borrows his mind

War turned the universal machine from a theorem into a weapon. On 4 September 1939, the day after Britain declared war on Germany, Turing reported to Bletchley Park and signed the Official Secrets Act. Within weeks he specified the Bombe, an electromechanical machine that searched Enigma's settings for contradictions against a guessed fragment of plaintext. He chose the hardest target, the German naval Enigma, "because no one else was doing anything about it and I could have it to myself," and in December 1939 cracked its indicator system and devised Banburismus, a statistical method that cut Bombe time by ruling out rotor orders.

The naval cipher guarded the U-boats, and the U-boats the Atlantic. When Bletchley fell behind, Turing was first named on a letter to Churchill for more staff and machines. Churchill's reply was a memo: "ACTION THIS DAY. Make sure they have all they want on extreme priority and report to me that this has been done." Turing's biographer Andrew Hodges writes that "this letter had an electric effect." For all of it he received the OBE, and a silence that lasted decades.

Here is the mechanism the state ran on him. It took his abstract machine and made it count U-boats. It bound him to secrecy so total that he could not tell anyone, including the courts that would judge him, what he had done for the country. And then, his usefulness spent, it filed him under a different heading. After 1943 he knew electronic hardware was fast enough to make a real universal machine, and he made building one his "principal ambition," a plan he called, in Hodges's phrase, "building a brain". His 1945 design for the Automatic Computing Engine was the first relatively complete specification of a stored-program electronic computer. A poor politician of his own ideas, he was sidelined at Manchester and ran marathons to near-Olympic standard while institutions built, slowly, what he had already imagined.

The conviction

The man in the Wilmslow police station was no nobody betting the law had moved on, but the state's most classified asset, and he had just handed the police a confession. At the committal on 27 February 1952 he was granted bail of fifty pounds; Murray was held in custody. Five weeks later, on 31 March 1952, at Knutsford in Cheshire, Regina v. Turing and Murray was heard. Both pleaded guilty on all six counts.

The trial had a moment of grace. His friend Max Newman, colleague from Bletchley and Manchester, was called as a character witness. "He is completely absorbed in his work, and is one of the most profound and original mathematical minds of his generation," Newman told the court. His barrister tried to keep him out of prison by pointing to the work: "There is treatment which could be given him. I ask you to think that the public interest would not be well served if this man is taken away from the very important work he is doing." None of it touched the verdict. The judge sentenced him to twelve months' probation and ordered him to "submit for treatment by a duly qualified medical practitioner at Manchester Royal Infirmary." The treatment was organotherapy, injections of synthetic oestrogen, a chemical castration that left him impotent and grew him breasts. He wrote to a friend on 17 April 1952: "It is supposed to reduce sexual urge whilst it goes on, but one is supposed to return to normal when it's over. I hope they're right." After sentencing he wrote, "No doubt I shall emerge from it all a different man, but quite who I've not found out," and signed the letter "Yours in distress."

In a few pages of unfinished fiction, a scientist called Alec Pryce, a thin disguise for the author, makes a discovery called "Pryce's buoy," a proxy for the universal machine, and "Alec always felt a glow of pride when this phrase was used," Turing wrote. "The rather obvious double-entendre rather pleased him too. He always liked to parade his homosexuality, and in suitable company Alec would pretend that the word was spelt without the 'u.'" The man who invented the universal machine knew what he was, knew the law criminalised it, and still told the police the truth. His honesty was the same instinct that made him swap foggy questions for operational tests. He would not live a lie.

The conviction cost him his security clearance and his GCHQ consultancy, Bletchley's post-war successor; he told a friend he would never work for them again. The timing was brutal: the Burgess and Maclean defection to the Soviet Union in 1951 had panicked the Foreign Office, and a known homosexual was now by policy a security risk. The country he had helped save reclassified him as a liability.

The apple, and the argument about it

On 7 June 1954, at his house in Wilmslow, Alan Turing died of cyanide poisoning. He was forty-one. His housekeeper found him in bed with a half-eaten apple on the bedside table. The coroner, Mr J. A. K. Ferns, recorded a verdict of suicide "while the balance of his mind was disturbed," and added, in a phrase that tells you more about 1954 than about Turing: "In a man of his type, one never knows what his mental processes are going to do next." The apple was never tested for cyanide.

The story has been told as a fairy-tale echo, the mathematician re-enacting Snow White's poisoned apple. It is a tidy story, which is the problem. Turing kept cyanide for home chemistry in a spare room he called the "nightmare room," electroplating spoons with gold. Jack Copeland argues the inquest would not today support a suicide verdict: Turing had been cheerful, the room smelled of cyanide, and the poison in his organs fits inhalation better than a bitten apple. "The exact circumstances of Turing's death will probably always be unclear," Copeland concludes. Hodges disagrees, reading the chemistry as a deliberate cover arranged so his mother could believe it was an accident. "I have no doubt at all," Hodges has said, "that this experiment was there as a cover which allowed people, especially his mother, to believe that it was an accident." Even the manner of his death is disputed, because the investigation was done badly by people who had decided what a man of his "type" was.

What is not disputed is what killed him: a state that used him, concealed him, and poisoned him by prescription for loving another man. The reversal was slow and official. In 2009, after a petition of more than thirty thousand signatures, Gordon Brown issued a formal apology. "It is no exaggeration to say," Brown wrote, "that, without his outstanding contribution, the history of World War Two could well have been very different." He ended: "So on behalf of the British government, and all those who live freely thanks to Alan's work I am very proud to say: we're sorry, you deserved so much better." In 2013, the Queen granted a rare royal pardon, only the fourth since the war. In 2017, the "Alan Turing Law" retroactively pardoned the tens of thousands of men convicted under the same gross-indecency statutes. The law that Turing, in the Wilmslow police station, had assumed was on its way out finally fell, sixty-five years too late for him.

He asked whether a machine could think, and replaced the bad question with a test we are still running. He invented a single machine that could be any machine, and you are using it now. He asked, at eighteen, whether his dead friend's mind would find "a new body sooner or later perhaps immediately," and spent his life building bodies for minds. The country he served broke him for honesty. The least we can do is get his life right.

Read the full life in Andrew Hodges's Alan Turing: The Enigma (Princeton University Press, 2014; first published 1983), the biography on which every later account, and the film The Imitation Game, rests.

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