Alan Turing: A Life in Five Chapters

Chapter 1 · The Boy Who Did the Wrong Homework

School, and a friend who died

1912 – 1931 · London · Sherborne · Cambridge

Alan Turing was born in London on 23 June 1912. His father worked in the Indian Civil Service, so Alan and his brother were left with a foster family in England while their parents were abroad — a common arrangement in imperial families and, by his biographers' accounts, an isolating one.

At Sherborne School he was a problem. The school prized classics and team sports; Turing solved advanced problems without being taught the intermediate steps and ignored subjects he found pointless. His headmaster wrote that if he was to stay at a public school he must aim at becoming educated, and that if he was to be solely a scientific specialist he was wasting his time. On his first day, during the 1926 General Strike, he cycled about sixty miles from Southampton to reach the school, staying overnight on the way.

The most important thing that happened to him there was Christopher Morcom, a brilliant fellow pupil and his first close friend and first love. Morcom died in February 1930 of complications from bovine tuberculosis contracted years earlier from infected milk. Turing was devastated, wrote to Morcom's mother for years, and turned over the question of whether a mind could exist apart from a body — a question that stayed with him and shaped his later interest in whether thinking could be separated from the stuff doing it.

He went to King's College, Cambridge, in 1931, took a first in mathematics, and was elected a fellow at twenty-two.

Why this matters

Turing's question after Morcom's death — whether mind is separable from matter — runs directly into his later work on whether a machine could think.

You have met the boy who did not fit his school. What would you ask him?

Ask Turing

• “Why did school find you so difficult?”
• “What did Christopher Morcom mean to you?”
• “Did you think a mind could exist without a body?”
• “Why did you cycle sixty miles to your first day of school?”
• “What did Cambridge give you that school could not?”

Chapter 2 · On Computable Numbers

Inventing the computer as a thought experiment

1936 – 1938 · Cambridge · Princeton

In 1936, aged twenty-three, Turing published a paper answering a question posed by David Hilbert: is there a definite procedure that can determine, for any mathematical statement, whether it is provable?

To answer it he had to define what a definite procedure is. His solution was to imagine the simplest possible machine: an infinite tape divided into squares, a head that reads and writes symbols, and a table of rules saying what to do given the current symbol and the machine's state. Anything a human could compute by following rules mechanically, he argued, such a machine could compute.

He then made the decisive move. Since the rules can themselves be written as symbols on the tape, one machine can be built to read the description of any other machine and imitate it. That is the universal Turing machine — the abstract idea of a general-purpose, programmable computer, described nine years before one was built.

With it he proved Hilbert's answer was no. Some problems are undecidable, including the halting problem: no general procedure can determine, for every program and input, whether that program will eventually stop.

The paper is thus simultaneously a limit on what mathematics can do and the blueprint for the machine that has done more than any other. Alonzo Church at Princeton had reached an equivalent conclusion by different means months earlier; Turing went to Princeton to work with him, took a doctorate in 1938, turned down a job with von Neumann, and came home.

Why this matters

The universal Turing machine is the theoretical foundation of computer science: every general-purpose computer is a physical approximation of it.

You have the idea of a computer before computers. What would you ask him?

Ask Turing

• “What is a Turing machine, in plain terms?”
• “Why does one machine being able to imitate any other matter so much?”
• “What is the halting problem?”
• “How did you arrive at such a simple model of computing?”
• “Did you realise you had described a real machine?”

Chapter 3 · Hut 8

Enigma, the Bombe, and a letter to Churchill

1939 – 1945 · Bletchley Park

Turing arrived at Bletchley Park the day after Britain declared war. The German Enigma machine enciphered messages with rotor settings changed daily, giving an astronomical number of possible configurations.

Polish mathematicians — Marian Rejewski, Jerzy Różycki and Henryk Zygalski — had already broken earlier Enigma traffic and built a machine called the *bomba*, and in July 1939 they handed everything to British and French intelligence. Bletchley's work started from theirs, a fact long omitted from British accounts.

Turing's Bombe, refined with Gordon Welchman's diagonal board, worked differently. Rather than searching for the key directly, it exploited *cribs* — guessed plaintext, such as routine weather reports — and tested settings by looking for logical contradictions, eliminating vast numbers of possibilities at once. He also developed Banburismus, a sequential statistical method for narrowing rotor settings that used ideas close to modern Bayesian inference.

He led Hut 8, which attacked naval Enigma — the hardest and the most urgent, since U-boats were sinking the convoys Britain depended on for food and fuel.

In October 1941, frustrated by shortages of staff and equipment, Turing and three colleagues wrote directly to Churchill. Churchill minuted 'Action this day', and the resources arrived.

Historians generally estimate that Bletchley's work shortened the war by a year or more and saved many hundreds of thousands of lives. Turing was appointed OBE in 1946 for services he could not describe to anyone, and the work stayed secret until the 1970s.

“Action this day.”

— Winston Churchill's minute on the letter from Turing, Welchman, Alexander and Milner-Barry, 21 October 1941

Why this matters

Bletchley's Enigma work is the clearest historical case of mathematics changing the outcome of a war — and the Polish contribution is a reminder to check whose work a national story leaves out.

You are inside the codebreaking. What would you ask him?

Ask Turing

• “How did the Bombe actually narrow down Enigma settings?”
• “What did the Polish codebreakers give you to start from?”
• “Why was naval Enigma the hardest problem?”
• “Why write directly to Churchill?”
• “What was it like knowing things you could tell no one?”

Chapter 4 · Can Machines Think?

The imitation game, and the spots on a leopard

1945 – 1952 · Teddington · Manchester

After the war Turing designed the Automatic Computing Engine at the National Physical Laboratory — a stored-program computer far more ambitious than anything then being built. Progress was slow and he left for Manchester, where he wrote software for one of the world's first working stored-program machines and produced a chess-playing program before any computer could run it, executing the moves by hand.

In 1950 he published 'Computing Machinery and Intelligence'. Rather than argue about what thinking is, he proposed a test: an interrogator communicates by text with a human and a machine and tries to tell which is which. If the machine cannot reliably be identified, he argued, the question of whether it 'really' thinks has lost its interest. He then set out and answered nine objections, including the theological one and the claim that machines can only do what we tell them.

The paper founded artificial intelligence as a field and remains the reference point for every argument about it.

His last major work went in a different direction. In 1952 he published 'The Chemical Basis of Morphogenesis', showing mathematically how two chemicals that react and diffuse at different rates can spontaneously produce stable patterns from an even starting state — stripes, spots, spirals. It explains how an embryo with no blueprint can develop structure. It was largely ignored for decades and is now foundational in mathematical biology; experimental confirmation of Turing patterns in real biological systems came in the twenty-first century.

He was, characteristically, using computers to run the numerical simulations.

Why this matters

Turing's 1950 paper set the terms of the debate about machine intelligence that societies are still having, seventy years later, with systems he never saw.

You have the questions he left for us. What would you ask him?

Ask Turing

• “Why replace 'can machines think' with a test?”
• “Which objection to machine intelligence did you find strongest?”
• “How can two chemicals produce a leopard's spots?”
• “Why did your biology paper go unnoticed for so long?”
• “What would you make of today's language models?”

Chapter 5 · 1952

A burglary, a conviction, and an apology sixty years late

1952 – 1954 and after · Manchester · Wilmslow

In January 1952 Turing's house was burgled. He reported it to the police. In the course of the investigation he described his relationship with Arnold Murray, a nineteen-year-old man connected to the burglar, and was charged with gross indecency under the same 1885 law used against Oscar Wilde.

He did not deny it or express regret. Convicted in March 1952, he was offered a choice between prison and probation conditional on hormone treatment — synthetic oestrogen injections, sometimes called chemical castration. He chose probation to keep working. The treatment caused physical changes and lasted a year.

The deeper damage was professional. His security clearance was revoked, ending consultancy for GCHQ, and he was barred from the United States. In Cold War Britain, homosexuality was treated as a blackmail risk, so the man who had helped break Enigma became, officially, a security liability.

On 8 June 1954 his housekeeper found him dead. The cause was cyanide poisoning and the inquest recorded suicide. A partly eaten apple was beside the bed and was never tested. His mother maintained it was an accident from a home electroplating experiment involving cyanide, and the historian Jack Copeland has argued the evidence does not settle the question. Suicide remains the most widely accepted account, but it is not proved beyond dispute.

He was forty-one.

In 2009 the British government issued a formal apology; in 2013 he received a royal pardon; and in 2017 the Policing and Crime Act, known informally as Turing's Law, extended pardons to thousands convicted under the same laws. He appears on the Bank of England fifty-pound note.

Why this matters

Turing's prosecution is the clearest single illustration of what anti-homosexuality laws cost — not only in individual suffering, but in the loss of a mind at the height of its powers.

You have the ending and the long apology. What would you ask him?

Ask Turing

• “Why did you tell the police the truth?”
• “Why choose hormone treatment over prison?”
• “What did losing your security clearance cost?”
• “How certain is it that your death was suicide?”
• “What does a pardon sixty years later actually mean?”

What Turing changed

Turing defined computation and described the universal programmable machine that all computers implement. His codebreaking work at Bletchley Park is credited with shortening the Second World War, his 1950 paper founded artificial intelligence as a field, and his work on morphogenesis became a foundation of mathematical biology.

A debate that continues

Historians debate the exact circumstances of his death, and the Polish mathematicians' foundational contribution to breaking Enigma is still under-acknowledged in popular British accounts.

Keep exploring — ask Turing

• “How do you build a machine to defeat a machine?”
• “What would convince you that something is thinking?”
• “What would you have worked on next?”

Related lives

• Ada Lovelace — Enchantress of Number · First Programmer
• Grace Hopper — Amazing Grace · Mother of COBOL
• Oscar Wilde — Wit · Aesthete · Poet

Related themes

Computer science and algorithms · Cryptography and the Second World War · Artificial intelligence