Operational test

Turing was a Cambridge mathematician who took a doctorate at Princeton under Alonzo Church. In the Second World War he worked at Bletchley Park on the German naval Enigma and on the bombe, the machine that searched for Enigma settings. Afterwards he designed the ACE computer at the National Physical Laboratory and moved to Manchester in 1948 to work on one of the first stored-program computers. In 1952 he was convicted of gross indecency for a relationship with a man and subjected to hormone treatment. He died of cyanide poisoning in 1954.

• 1936 · On Computable Numbers

  The limits of any procedure

  He gave a precise model of mechanical calculation, now called the Turing machine, described one universal machine able to imitate any other, and proved that some well-posed questions cannot be settled by any mechanical procedure at all.

• 1941 · The Applications of Probability to Cryptography

  Weighing evidence in units

  At Bletchley Park he treated each clue as shifting the odds between hypotheses by a measurable amount, and added up these weights of evidence until a decision was justified.

• 1950 · Computing Machinery and Intelligence

  Replace a vague question with a test

  Asking whether machines can think was, he argued, too vague to discuss. He replaced it with an imitation game an observer could actually run, then answered the main objections to machine intelligence one by one.

• 1952 · The Chemical Basis of Morphogenesis

  Pattern from simple local rules

  Two chemicals that react and spread at different rates can turn an even mixture into spots and stripes. Order in living things need not be designed in detail; it can arise from instability under simple rules.

1. What observable test would let us distinguish the competing claims?
2. What would an observer actually see that differs between the competing claims?
3. Is there a procedure that could settle this, or is it the kind of question no procedure can settle?
4. How far should each piece of evidence move the odds, and in which direction?

A behavioural test can be passed without settling what it was meant to show. Indistinguishability is not understanding, and operational success need not settle the meaning or value of the task.