The centenary of Alan Turing’s birth is being greeted by an extraordinary response, not only in mathematical and scientific circles but in a much wider public arena. It marks the awareness that he was one of the 20th century’s seminal figures, whose brief life is better appreciated in the 21st century than in his own.

One reason for this fascination is that he was an unworldly person at the heart of an amazingly worldly achievement: the breaking of Nazi Germany’s most closely guarded military ciphers. Entirely unknown to the public in his own time, his tour de force emerged only in the 1970s. (Even in 2012, his wartime papers are still being declassified, and the full story remains unwritten.)

Yet this triumph of reason is eclipsed by the fundamental discovery Turing made in 1936. At 24, he proved something very strange: There are tasks that can be described but cannot be carried out by a definite procedure, method, or process. This was his negative conclusion, but in order to establish it, he had to do something very constructive. He had to define what was meant by a definite procedure, method, or process. No one had ever done that, and Turing’s solution came from the future.

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His “Turing machine,” then a new mathematical construct, is now easily seen as the abstract form of a computer program. In 1936, computers did not exist. But Turing defined the concept of the “universal machine,” with the power to run any program. In so doing he invented the concept of the computer.

Turing’s most radical idea resided within that invention. It was essential to his argument that instructions could be seen as a form of data. In modern computing, that is a commonplace: Programs act on programs. But in the 1930s, it was a strange and radical step, flowing from the abstruse and abstract logic of Turing’s mathematical proof.

In 1945, fully aware of the power of electronic technology being used in wartime code breaking, Turing set out to turn his idea of a universal machine into an electronic digital computer. More important than the details of his hardware engineering design is the prospectus he launched for computer software, capable of encompassing “every known process” and embodying the modern logic of programs working on other programs.

Turing called his project “building a brain,” and indeed, even from 1936, Turing’s interests centered on modeling the human mind. His later 1940s work developed ideas for artificial intelligence, and in 1950 he went on to found a completely new field in mathematical biology, using computer simulation to study growth and form in plants and animals. But his fascination with the brain reemerged in his famous paper of 1950, “Computing Machinery and Intelligence,” which explains the so-called Turing Test for machine-based intelligence.

The paper has attracted a wide readership, not just for its bold argument but for its wit and humor. Turing’s subtext was a vigorous statement that he was entirely human.

The success of this publication was at odds with his own reticence to claim the origin of the computer as his own. He did little to attach his name to it and in his own lifetime was written out of the story.

Even so, the revelations of his wartime triumph have rescued the remarkable accomplishments of an unusual genius who combined the highly abstract with the hands-on approach of an engineer. His modern-minded openness as a gay man, one who suffered prosecution and punishment in 1952, has also attracted great attention. Alan Turing is a hero of the theory and practice of computer science. Adding his roles in the human dramas of war and sexuality, he has achieved a special place in history.

## About the Author

Andrew Hodges is a fellow in mathematics at the University of Oxford and the author of the 1983 biography *Alan Turing: The Enigma*, which was reissued this year by Princeton University Press.

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