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Alan Turing
Computing Machinery and Intelligence

Squashed down to read in about 25 minutes
"Can machines think?"

Turing and the 'Manchester Baby' computer he helped create

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  INTRODUCTION TO Computing Machinery and Intelligence

Alan Turing is the 'Father of The Computer'- a brilliant original thinker who studied subjects from philosophy and psychology through to physics, chemistry and biology. The word repeatedly used is 'genius'. It was in the 1930's that he first conceived of a 'universal machine' - a machine unlike any imagined before- one which could carry out any logical process without altering the machine itself. He suggested that such a machine should be called a computer.

During Hitler's war his part in building the proto-computer 'Bombe' device allowed German wartime codes to be cracked and after, in Manchester, he contributed to building and programming the first all-electronic computer, and began the study of artificial intelligence.

In 1952 he was prosecuted for the crime of homosexuality. He was excluded from the work he loved, and, aged just 42, died after taking a bite from an apple poisoned with cyanide. His work on logical systems remains unfinished and is still being studied.

His proposal here for the 'Imitation Game' as a test for computers is now known as the 'Turing Test'

(Note: Up to Turing's time the word 'computer' referred to person skilled at calculation, not to a machine.)


Turing's paper was first published in 'Mind' 1950 (vol 59, p433-460). The text beside Turing's portrait is a facsimile of what might be described as 'the first e-mail'- sent by Turing from the Manchester 'Baby' computer to (there not being another computer) a telex machine. Note that, after just 7 lines, the pioneer computer had run out of memory and Turing had to write-in the last few words by hand.


Alan Turing,
Computing Machinery and Intelligence

"Can machines think?"

"Can machines think?" The problem can be described in terms of the "imitation game", with a man, a woman, and an interrogator. The interrogator stays in a room apart front the other two and tries, by sending questions (perhaps by teleprinter) to the others, to determine which is the man and which the woman. Could a digital computer convince the interrogator that it was a man?
Digital computers work by following written-down rules, called 'programmes', and can even change their own rules in response to other rules. I believe that in about fifty years' time it will be possible to programme computers to make them play the imitation game.
It could be argued that machines cannot have souls, but, if this matters at all, why could not God give a soul to whatever He wants. We cannot know if computers could have consciousness, because we cannot really know if other people have consciousness. Computers can still surprise us with their answers. It might be best to try and build a computer like a human infant, and then programme it to learn.

  The Squashed Philosophers Edition of...

Computing Machinery and Intelligence
Alan Turing

Squashed version edited by Glyn Hughes © 2011

1. The Imitation Game
I propose to consider the question, "Can machines think?" The problem can be described in terms of the "imitation game." It is played with three people, a man (A), a woman (B), and an interrogator (C). The interrogator stays in a room apart front the other two. The object of the game for the interrogator is to determine which of the other two is the man and which is the woman by putting questions to A and B thus:

C: Will X please tell me the length of his or her hair?

Now suppose X is actually A, then A must answer. It is A's object in the game to try and cause C to make the wrong identification. His answer might therefore be:

"My hair is shingled, and the longest strands are about nine inches long."

In order that tones of voice may not help the interrogator, the ideal arrangement is to have a teleprinter communicating between the two rooms. We now ask the question, "What will happen when a machine takes the part of A in this game?" Will the interrogator decide wrongly as often when the game is played like this as he does when the game is played between a man and a woman?

2. Critique of the New Problem
The new problem has the advantage of drawing a fairly sharp line between the physical and the intellectual capacities of a man. No engineer or chemist claims to be able to produce a material indistinguishable from human skin. Even supposing this invention were available we should feel there was little point in trying to make a "thinking machine" more human by dressing it up in such artificial flesh. Some advantages of the proposed criterion may be shown up by specimen questions and answers. Thus:

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.
Q: Add 34957 to 70764.
A: (Pause about 30 seconds and then give as answer) 105621.
Q: Do you play chess?
A: Yes.
Q: I have K at my K1, and no other pieces. You have only K at K6 and R at R1. It is your move. What do you play?
A: (After a pause of 15 seconds) R-R8 mate.

The game may perhaps be criticised on the ground that the odds are weighted too heavily against the machine. If the man were to try and pretend to be the machine he would clearly make a very poor showing. He would be given away at once by slowness and inaccuracy in arithmetic. May not machines carry out something which ought to be described as thinking but which is very different from what a man does?

3. The Machines Concerned in the Game
There are already a number of digital computers in working order, and it may be asked, "Why not try the experiment straight away? The short answer is that we are not asking whether the computers at present available would do well, but whether there are imaginable computers which would do well.

4. Digital Computers
The idea behind digital computers may be explained by saying that these machines are intended to carry out any operations which could be done by a human computer. The human computer is supposed to be following fixed rules; he has no authority to deviate from them in any detail. We may suppose that these rules are supplied in a book, which is altered whenever he is put on to a new job. He has also an unlimited supply of paper on which he does his calculations.

A digital computer can usually be regarded as consisting of three parts:

(i) Store.
(ii) Executive unit.
(iii) Control.

The store is a store of information, and corresponds to the human computer's paper, whether this is the paper on which he does his calculations or that on which his book of rules is printed. The executive unit is the part which carries out operations such as "Multiply 3540675445 by 7076345687", but in some machines only very simple ones such as "Write down 0" are possible.

The "book of rules" supplied to the computer is replaced in the machine by a part of the store. It is then called the "table of instructions." It is the duty of the control to see that these instructions are obeyed correctly and in the right order. The information in the store is usually broken up into packets of moderately small size. In one machine, for instance, a packet might consist of ten decimal digits. Numbers are assigned to the parts of the store in which the various packets of information are stored, in some systematic manner. A typical instruction might say-

"Add the number stored in position 6809 to that in 4302 and put the result back into the latter storage position."

Needless to say it would not occur in the machine expressed in English. It would more likely be coded in a form such as 6809430217. Here 17 says which of various possible operations is to be performed on the two numbers.

The control will normally take the instructions to be obeyed in the order of the positions in which they are stored, but occasionally an instruction such as

"Now obey the instruction stored in position 5606, and continue from there"

Instructions of this type are very important because they make it possible for a sequence of operations to be replaced over and over again until some condition is fulfilled. To take a domestic analogy. Suppose Mother wants Tommy to call at the cobbler's every morning on his way to school to see if her shoes are done, she can ask him afresh every morning. Alternatively she can stick up a notice once and for all in the hall which he will see when he leaves for school and which tells him to call for the shoes, and also to destroy the notice when he comes back with the shoes.

If one wants to make a machine mimic the behaviour of the human computer in some complex operation one has to ask him how it is done, and then translate the answer into the form of an instruction table. Constructing instruction tables is usually described as "programming."

An interesting variant is a "digital computer with a random element." Sometimes such a machine is described as having free will (though I would not use this phrase myself). It is not normally possible to determine from observing a machine whether it has a random element, for a similar effect can be produced by such devices as making the choices depend on the digits of the decimal for pi.

Most actual digital computers have only a finite store. There is no theoretical difficulty in the idea of a computer with an unlimited store. Such computers have special theoretical interest and will be called infinitive capacity computers.

The idea of a digital computer is an old one. Charles Babbage planned such a machine, called the Analytical Engine, but it was never completed. The fact that Babbage's Analytical Engine was to be entirely mechanical will help us to rid ourselves of a superstition. Importance is often attached to the fact that modern digital computers are electrical, and that the nervous system also is electrical. 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.

5. Universality of Digital Computers
The digital computer may be classified amongst the "discrete-state machines." These are the machines which move by sudden jumps or clicks from one quite definite state to another.

It will seem that given the initial state of the machine and the input signals it is always possible to predict all future states, This is reminiscent of Laplace's view that from the complete state of the universe at one moment of time, as described by the positions and velocities of all particles, it should be possible to predict all future states. The system of the "universe as a whole" is such that quite small errors in the initial conditions can have an overwhelming effect later. The displacement of a single electron by a billionth of a centimetre at one moment might make the difference between a man being killed by an avalanche a year later, or escaping. It is an essential property of the mechanical systems which we have called "discrete-state machines" that this phenomenon does not occur. Even when we consider the actual physical machines instead of the idealised machines, reasonably accurate knowledge of the state at one moment yields reasonably accurate knowledge any number of steps later.

Digital computers fall within the class of discrete-state machines. But the number of states of which such a machine is capable is usually enormously large. For instance, the number for the machine now working at Manchester is about 10^50,000. The special property of digital computers, that they can mimic any discrete-state machine, is described by saying that they are universal machines, with the important consequence that, considerations of speed apart, it is unnecessary to design various new machines to do various computing processes. They can all be done with one digital computer, suitably programmed for each case. It will be seen that as a consequence of this all digital computers are in a sense equivalent.

6. Contrary Views on the Main Question
I believe that in about fifty years' time it will be possible to programme computers to make them play the imitation game so well that an average interrogator will not have more than 70 per cent chance of making the right identification after five minutes of questioning. I now proceed to consider opinions opposed to my own.

(1) The Theological Objection
Thinking is a function of man's immortal soul. God has given an immortal soul to every man and woman, but not to any other animal or to machines. Hence no animal or machine can think. I am unable to accept any part of this, but will attempt to reply in theological terms.

The arbitrary character of the orthodox view becomes clearer if we consider how it might appear to a member of some other religious community. How do Christians regard the Moslem view that women have no souls? It appears to me that the argument above implies a serious restriction of the omnipotence of the Almighty. It is admitted that there are certain things that He cannot do such as making one equal to two, but should we not believe that He has freedom to confer a soul on an elephant if He sees fit? An argument of exactly similar form may be made for the case of machines.

(2) The "Heads in the Sand" Objection
"The consequences of machines thinking would be too dreadful. Let us hope and believe that they cannot do so."

We like to believe that Man is in some subtle way superior to the rest of creation. I do not think that this argument is sufficiently substantial to require refutation. Consolation would be more appropriate: perhaps this should be sought in the transmigration of souls.

(3) The Mathematical Objection
There are a number of results of mathematical logic which can be used to show that there are limitations to the powers of discrete-state machines. The best known of these is known as Godel's theorem (1931) and shows that in any sufficiently powerful logical system statements can be formulated which can neither be proved nor disproved within the system. The short answer to this argument is that although it is established that there are limitations to the powers of any particular machine, it has been stated, without any proof, that no such limitations apply to the human intellect. This feeling is no doubt quite genuine, but I do not think too much importance should be attached to it. We too often give wrong answers to questions ourselves to be justified in being very pleased at such evidence of fallibility on the part of the machines. Further, our superiority can only be felt on such an occasion in relation to the one machine over which we have scored our petty triumph. There would be no question of triumphing simultaneously over all machines. In short, then, there might be men cleverer than any given machine, but then again there might be other machines cleverer again, and so on.

(4) The Argument from Consciousness
This argument is well expressed by Professor Jefferson "Not until a machine can write a sonnet or compose a concerto because of thoughts and emotions felt, and not by the chance fall of symbols, could we agree that machine equals brain-that is, not only write it but know that it had written it. No mechanism could feel (and not merely artificially signal, an easy contrivance) pleasure at its successes, grief when its valves fuse, be warmed by flattery, be made miserable by its mistakes."

According to the most extreme form of this view, the only way by which one could be sure that machine thinks is to be the machine and to feel oneself thinking. One could then describe these feelings to the world, but of course no one would be justified in taking any notice. Likewise according to this view the only way to know that a man thinks is to be that particular man. The solipsist point of view may be the most logical view, but it makes communication of ideas difficult. Instead of arguing continually over this point it is usual to have the polite convention that everyone thinks. The game (with the player B omitted) is frequently used in practice under the name of viva voce to discover whether some one really understands something or has "learnt it parrot fashion." Let us listen in to a part of such a viva voce:

Interrogator: In the first line of your sonnet which reads "Shall I compare thee to a summer's day," would not "a spring day" do as well or better?
Witness: It wouldn't scan.
Interrogator: How about "a winter's day," That would scan all right.
Witness: Yes, but nobody wants to be compared to a winter's day.
Interrogator: Would you say Mr. Pickwick reminded you of Christmas?
Witness: In a way.
Interrogator: Yet Christmas is a winter's day, and I do not think Mr. Pickwick would mind the comparison.
Witness: I don't think you're serious. By a winter's day one means a typical winter's day, rather than a special one like Christmas.

And so on, What would Professor Jefferson say if the sonnet-writing machine was able to answer like this in the viva voce? I do not know whether he would regard the machine as "merely artificially signalling" these answers, but if the answers were as satisfactory and sustained as in the above passage I do not think he would describe it as "an easy contrivance."

I do not wish to give the impression that I think there is no mystery about consciousness. There is, for instance, something of a paradox connected with any attempt to localise it. But I do not think these mysteries necessarily need to be solved before we can answer the question with which we are concerned in this paper.

(5) Arguments from Various Disabilities
These arguments take the form, "I grant you that you can make machines do all the things you have mentioned but you will never be able to make one to do X." Be kind, resourceful, friendly, have initiative, have a sense of humour, make mistakes, fall in love, enjoy strawberries and cream, learn from experience, use words properly, be the subject of its own thought, do something really new.

The inability to enjoy strawberries and cream may have struck the reader as frivolous. Possibly a machine might be made to enjoy this delicious dish, but any attempt to make one do so would be idiotic. What is important about this disability is that it contributes to some of the other disabilities, eg., to the difficulty of the same kind of friendliness occurring between man and machine as between white man and white man, or between black man and black man.

The claim that "machines cannot make mistakes" seems a curious one. One is tempted to retort, "Are they any the worse for that?" The claim that a machine cannot be the subject of its own thought can of course only be answered if it can be shown that the machine has some thought with some subject matter. If, the machine was trying to find a solution of the equation 2x - 40x - 11 = 0 one would be tempted to describe this equation as part of the machine's subject matter at that moment. In this sense a machine undoubtedly can be its own subject matter. The criticism that a machine cannot have much diversity of behaviour is just a way of saying that it cannot have much storage capacity.

These criticisms are often disguised forms of the argument from consciousness

(6) Lady Lovelace's Objection
Our most detailed information of Babbage's Analytical Engine comes from a memoir by Lady Lovelace (1842). In it she states, "The Analytical Engine has no pretensions to originate anything. It can do whatever we know how to order it to perform" (her italics). A variant of Lady Lovelace's objection states that a machine can "never do anything really new." A better variant of the objection says that a machine can never "take us by surprise."

Machines take me by surprise with great frequency. Perhaps I say, "I suppose the Voltage here ought to he the same as there." Naturally I am often wrong, and the result is a surprise for me. The view that machines cannot give rise to surprises is due, I believe, to a fallacy to which philosophers and mathematicians are particularly subject. This is the assumption that as soon as a fact is presented to a mind all consequences of that fact spring into the mind simultaneously with it. It is a very useful assumption under many circumstances, but one too easily forgets that it is false.

(8) The Argument from Informality of Behaviour
It is not possible to produce a set of rules purporting to describe what a man should do in every conceivable set of circumstances. One might for instance have a rule that one is to stop when one sees a red traffic light, and to go if one sees a green one, but what if by some fault both appear together? One may decide that it is safest to stop. But some further difficulty may well arise from this decision . To attempt to provide rules of conduct to cover every eventuality appears to be impossible. With all this I agree.

From this it is argued that we cannot be machines. We can demonstrate more forcibly that any such statement would be unjustified. For suppose we could be sure of finding such laws if they existed. Then given a discrete-state machine it should certainly be possible to discover by observation sufficient about it to predict its future behaviour, and this within a reasonable time, say a thousand years.

(9) The Argument from Extrasensory Perception
I assume that the reader is familiar with the idea of extrasensory perception, and the meaning of telepathy, clairvoyance, precognition and psychokinesis. These disturbing phenomena seem to deny all our usual scientific ideas. How we should like to discredit them! Unfortunately the statistical evidence, at least for telepathy, is overwhelming. Once one has accepted them it does not seem a very big step to believe in ghosts and bogies.

If telepathy is admitted it will be necessary to tighten our test up. To put the competitors into a "telepathy-proof room" would satisfy all requirements.

7. Learning Machines
In the process of trying to imitate an adult human mind we are bound to think a good deal about the process which has brought it to the state that it is in. We may notice three components.

(a) The initial state of the mind, say at birth,
(b) The education to which it has been subjected,
(c) Other experience, not to be described as education, to which it has been subjected.

Instead of trying to produce a programme to simulate the adult mind, why not rather try to produce one which simulates the child's? Our hope is that there is so little mechanism in the child brain that something like it can be easily programmed.

We normally associate punishments and rewards with the teaching process. Some simple child machines can be constructed or programmed on this sort of principle. The idea of a learning machine may appear paradoxical to some readers. How can the rules of operation of the machine change? The explanation is that the rules which get changed in the learning process are of a rather less pretentious kind, claiming only an ephemeral validity. The reader may draw a parallel with the Constitution of the United States.

It is probably wise to include a random element in a learning machine, this is rather useful when we are searching for a solution of some problem. Suppose we wanted to find a number between 50 and 200 which was equal to the square of the sum of its digits, we might start at 51 then try 52 and go on until we got a number that worked.

Now the learning process may be regarded as a search for a form of behaviour which will satisfy the teacher. Since there is probably a very large number of satisfactory solutions the random method seems to be better than the systematic. We may hope that machines will eventually compete with men in all purely intellectual fields. But which are the best ones to start with? Even this is a difficult decision. Many people think that a very abstract activity, like the playing of chess, would be best. It can also be maintained that it is best to provide the machine with the best sense organs that money can buy, and then teach it to understand and speak English. This process could follow the normal teaching of a child. Things would be pointed out and named, etc. Again I do not know what the right answer is, but I think both approaches should be tried.

We can only see a short distance ahead, but we can see plenty there that needs to be done.

Alan Turing
The Turing Memorial in Sackville Park, Manchester
(Sculpted by the SqaPo Editor, Glyn Hughes) More...

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