Walker Publishing Company 2000. ISBN 0-8027-7580-2

Turings name has been kept alive by the successful "Turing Test" meme, the idea being that, "If a computer, on the basis of its written replies to questions, could not be distinguished from a human respondent, then "fair play" would oblige one to say that it must be thinking." - a lovely operational definition.

It turns out that the "Turing Test" was very typical of his line of thought. He was a mathematician but an experimenter at heart. At the age of 12 in France he was extracting iodine from seaweed with a newly aquired chemical set. He later told his mother that he had had his eyes opened to science by an American book "Natural Wonders Every Child Should Know" that he read when he was 10.

He was academically successful although something of a social outsider, going to Kings College Cambridge and being elected a Fellow at the unusually early age of 22. He was taught maths by M.H.A.Newman who had attended the 1928 international conference at which Hilbert had among other things asked whether maths was decidable, i.e. was there a definite method which could in principle be applied to any assertion and which was guaranteed to produce a correct decision as to whether that assertion was true.

Turing picked up on Newmans idea of a mechanical process as the answer.

In effect he went on to build a computer on paper: Read a tape of blank spaces and dots from left to right. If you find a dot do one thing, if you find a blank space do another - with this including moving to a new set of instructions about what to do when you meet the next blank space or dot. It was easy to put the instructions in an IF, THEN table nicely illustrated on page 99. The table is the machine.

The important point was that the "machine" was reading and (top)

making decisions.

In 1935-36 he was following up his ideas in the preparation of his paper, "Computable Numbers". As Hodges puts it, "..the Turing machine opened the door to a new branch of deterministic science. It was a model on which the most complex procedures could be built out of the elementary bricks of states and positions, reading and writing. It suggested a wonderful mathematical game, that of expressing any "definite method" whatever in a standard form."

At the outbreak of war his commanding officer wrote that "..we have been obliged to recruit from our emergency list men of the Professor type..." one of which was Turing.

The story changes here as Hodges very effectively explains how Alan Turing and his colleagues (particularly Gordon Welchman) develop logic machines that move from electro-magnetic to electronic switching (credit to Post Office engineer T.E.Flowers here) that eventually defeat the increasingly complex German "Enigma" cypher machine. The end of the effort is "Colossus", the absolutely secret first large electronic computer.

At the end of the book Hodges says about Turing that, "He was quite serious in describing the "almost certain" advent of intelligent machines as a development "which can give us anxiety"." but he adds that, "there seems every reason to suppose that the clever machine will accomodate itself to the crazy demands of the political system in which it is embodied."

One minor criticism is that Turing was a homosexual and Hodges (who is also homosexual) tends in my opinion to exagerate the influence that this had on his work. Nevertheless it is very sympathetic writing and it is possible that were it not for this link this truly excellent biography may never have been written.

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