Alan Turing's greatest achievement wasn't cracking Enigma

Ask most people what Alan Turing is known for, and the answer is usually his work at Bletchley Park cracking Nazi Germany's Enigma cipher during the Second World War. But a view increasingly shared among historians holds that Turing's most lasting legacy lies elsewhere, in the foundations he laid for theoretical computer science before the war even began.

Turing's most significant contribution in that field traces back to a paper he published in 1936, before the war started. Titled "On Computable Numbers," the paper introduced a theoretical concept now known as the Turing machine, an abstract computing model capable of reading and writing symbols on a tape according to a simple set of rules.

The Turing machine was never a physical device but a mathematical thought experiment; even so, the concept precisely defined the very idea of "computation" and showed which problems could, in principle, be solved by a machine and which could not. Today, every student of computer science traces the theoretical foundation of modern computers back to this concept.

The codebreaking work at Bletchley Park was, of course, hugely significant to the course of the war, and Turing's contributions there are extensively documented. But historians note that, despite its strategic importance, that work did not create as lasting an impact as the door opened by his 1936 theoretical paper, since cracking Enigma solved a concrete problem of its era, while the Turing machine concept became the foundation of the entire computing industry in the decades that followed.

After the war, Turing worked on early computer design at the National Physical Laboratory and later at the University of Manchester, work that helped translate his theoretical concepts into practical machines.

In 1950, Turing published a paper titled "Computing Machinery and Intelligence," in which he proposed what is now known as the Turing test as a way of exploring the question of whether a machine can "think." The paper is regarded as one of the philosophical and conceptual foundations of the field of artificial intelligence, made all the more remarkable for having been written when computers were still in their infancy.

In the final years of his life, Turing turned his attention to mathematical biology, working to explain pattern formation in living organisms, such as how the spots or stripes on an animal's skin emerge, through mathematical models. This work is considered pioneering in the field of morphogenesis and was later experimentally validated by biologists.

Turing's personal life intersected tragically with the legal environment of his time: in 1952, he was prosecuted and convicted under a law that criminalised homosexual acts in Britain at the time. That conviction caused serious damage to his career, including the revocation of his security clearance. Turing died in 1954.

The British government issued a formal apology for its treatment of Turing in 2009, and the Crown granted him a posthumous royal pardon in 2013. A law that came into force in 2017, covering thousands of other men convicted under similar historical offences, is popularly known as the "Alan Turing law."

Among historians and computer scientists today, a common view has emerged: if cracking Enigma made Turing a wartime hero, his 1936 theoretical work and his 1950 paper on artificial intelligence made him one of the intellectual architects of the modern computing era, a legacy that traces directly to today's computing and AI technologies.