The Information - James Gleick [121]
But what if the question is not which gender but which genus: human or machine?
It is understood that the essence of being human lies in one’s “intellectual capacities”; hence this game of disembodied messages transmitted blindly between rooms. “We do not wish to penalise the machine for its inability to shine in beauty competitions,” says Turing dryly, “nor to penalise a man for losing in a race against an aeroplane.” Nor, for that matter, for slowness in arithmetic. Turing offers up some imagined questions and answers:
Q: Please write me a sonnet on the subject of the Forth Bridge.
A: Count me out on this one. I never could write poetry.
Before proceeding further, however, he finds it necessary to explain just what sort of machine he has in mind. “The present interest in ‘thinking machines,’ ” he notes, “has been aroused by a particular kind of machine, usually called an ‘electronic computer’ or ‘digital computer.’ ”♦ These devices do the work of human computers, faster and more reliably. Turing spells out, as Shannon had not, the nature and properties of the digital computer. John von Neumann had done this, too, in constructing a successor machine to ENIAC. The digital computer comprises three parts: a “store of information,” corresponding to the human computer’s memory or paper; an “executive unit,” which carries out individual operations; and a “control,” which manages a list of instructions, making sure they are carried out in the right order. These instructions are encoded as numbers. They are sometimes called a “programme,” Turing explains, and constructing such a list may be called “programming.”
The idea is an old one, Turing says, and he cites Charles Babbage, whom he identifies as Lucasian Professor of Mathematics at Cambridge from 1828 to 1839—once so famous, now almost forgotten. Turing explains that Babbage “had all the essential ideas” and “planned such a machine, called the Analytical Engine, but it was never completed.” It would have used wheels and cards—nothing to do with electricity. The existence (or nonexistence, but at least near existence) of Babbage’s engine allows Turing to rebut a superstition he senses forming in the zeitgeist of 1950. People seem to feel that the magic of digital computers is essentially electrical; meanwhile, the nervous system is also electrical. But Turing is at pains to think of computation in a universal way, which means in an abstract way. He knows it is not about electricity at all:
Since Babbage’s machine was not electrical, and since all digital computers are in a sense equivalent, we see that this use of electricity cannot be of theoretical importance.… The feature of using electricity is thus seen to be only a very superficial similarity.♦
Turing’s famous computer was a machine made of logic: imaginary tape, arbitrary symbols. It had all the time in the world and unbounded memory, and it could do anything expressible in steps and operations. It could even judge the validity of a proof in the system of Principia Mathematica. “In the case that the formula is neither provable nor disprovable such a machine certainly does not behave in a very satisfactory manner, for it continues to work indefinitely without producing any result at all, but this cannot be regarded as very different from the reaction of the mathematicians.”♦ So Turing supposed it could play the Imitation Game.
He could not pretend to prove that, of course. He was mainly trying to change the terms of a debate he considered largely fatuous. He offered a few predictions for the half century to come: that computers would have a storage capacity of 109 bits (he imagined a few very large computers; he did not foresee our future of ubiquitous tiny computing devices with storage many magnitudes greater than that); and that