Alan Turing

Alan Turing occupies a singular place: he is at once the theoretical founder of computer science and the first to have posed the question of machine intelligence in terms that still structure our debates — seventy-five years later.

From the paper machine to the computer

In 1936, at the age of twenty-four, Turing published “On Computable Numbers”. To solve a problem in mathematical logic, he invented a paper machine: an abstract automaton that reads and writes symbols on a tape according to simple rules. He demonstrated that a single machine of this kind — the universal machine — can carry out any computation, provided it is given the description of it. The modern computer is the material realisation of this idea: our machines are not sophisticated calculators, they are universal machines.

During the Second World War, Turing put this power of abstraction to concrete use: at Bletchley Park, his work on breaking Enigma helped to shorten the conflict. After the war, he took part in the design of the first electronic computers, before turning to mathematical biology and the forms of living things.

The imitation game

In 1950, in the philosophy journal Mind, Turing published “Computing Machinery and Intelligence”. The question “can machines think?” struck him as badly posed — too laden with metaphysics. In its place he put a game: an interrogator converses in writing with two hidden interlocutors, a human and a machine. If the interrogator cannot tell them apart, what remains of the idea that the machine does not think?

In the same article, Turing anticipates and discusses nine objections, among them Ada Lovelace's: a machine only does what it is ordered to do, it creates nothing. Turing replies that this argument holds for fixed-program machines — not for learning machines, capable of surprising their own designer. The reply, formulated half a century before machine learning, has since taken on a singular resonance.

A question still open

Turing defended a continuity between the human and the machine: intelligence would be a function, not a substance; the difference, a matter of degree and not of nature. One may contest this position — a whole philosophical tradition sets about doing so. But it was he who fixed the terms of the discussion: since conversational systems began sustaining dialogues hard to distinguish from those of a human, his test has left the laboratories to become an everyday experience — and the question of what, exactly, it proves has never been so alive.

Persecuted by the British justice system for his homosexuality, sentenced to chemical castration in 1952, Turing died two years later, at the age of forty-one. His posthumous rehabilitation does not efface what his death cost to thought.