Chapter 4: Computing Machinery And Intelligence

The Imitation Game

I propose to consider the question “Can machines think?” This should begin with definitions of the terms “machine” and “think.” The definitions might be framed so as to reflect as far as possible the normal use of the words, but this attitude is dangerous. If the meaning of the words “machine” and “think” are to be found by examining how they are commonly used it is difficult to escape the conclusion that the meaning an dthe answer to the question “Can machines think?” is to be sought in a statistical survey such as a Gallop poll. But this is absurd. Instead of attempting such a definition I shall replace the question by another, which is closely related to it, and is expressed in relatively unambiguous words.
The new form of the problem can be described in terms of a game which we call the “imitation game.” It is played with three people, a man (A), a woman (B), and an interrogator (C) who may be of either sex. The interrogator stays in a room apart from the other two. The object of the game for the interrogator is to determine which of the other two is

Excerpt from “Computing Machines and Intelligence.” Mind, Vol. LIX. No. 236 1950). Reprinted by permission.

The man and which is the woman. He knows them by labels X, and Y, and at the end of the game he says either “X is A and Y is B” or “X is B and Y is A.” The interrogator is allowed to put 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 to 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 answers should be written, or better still, typewritten. The ideal arrangement is to have a teleprinter communicating between the two rooms. Alternatively the question and answers can be repeated by an intermediary. The object of the game is for the third player (B) is to help the interrogator. The best strategy for he is probably to give truthful answers. She can add such things as ”I am the woman, don’t listen to him!” to her answers, but it will avail nothing as the man can make similar comments.
We now ask the question, “What will happen when a machine take sthe part of A in this game?” Will the interrogator deicde wrongly as often when the game is played like this as he does when the game is played between a man and a woman? These questions replace our original “Can machines think?”


Critique of the New Problem

As well as asking “What is the answer to this new form of the question,” one may ask, “Is this new question a worthy one to investigate?” This latter question we investigate without further ado, thereby cutting short an infinite regress.
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 which is indistinguishable from the human skin. It is possible that at some time this might be done, but even supposing this invention 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. The form in which we have set the problem reflects this fact in the condition which prevents the interrogator from seeing or touching the other competitors, or from hearing their voices.

Some other 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 78764
A: (pause about 30 seconds and then give an 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 question and answer method seems to be suitable for introducing almost any one of the fields of human endeavour that we wish to include. We do not wish to penalize the machine for its inability to shine in beauty competitions, nor to penalize a man for losing in a race against an airplane. The conditions of our game make these disabilities irrelevant. The “witnesses” can brag, if they consider it advisable, as much as they please about their charms, strength or heroism, but the interrogator cannot demand practical demonstrations.
The game may perhaps be criticized 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? This objections is a very strong one, but at least we can say that if, nevertheless, a machine can be constructed to play the imitation game satisfactorily we need not be troubled by this objection.
It might be urged that when playing the “imitation game” the best strategy for the machine may possibly be something other than imitation of the behaviour of a man. This may be, but I think it is unlikely that there is any great effect of this kind. In any case there is no intention to investigate here the theory of the game, and it will be assumed that the best strategy is to try to provide answers that would naturally be given by a man.

The Machines Concerned in the Game

The question which we put earlier will not be quite definite until we have specified what we mean by the word “machine.” It is natural that we should wish to permit every kind of engineering technique to be used in our machines. We also wish to allow te possibility that an engineer or team of engineers may construct a machine which works, but whose manner of operation cannot be satisfactorily described by its constructors because they have applied a method which is largely experimental. Finally, we wish to exclude from the machines men born in the usual manner. It is difficult to frame the definitions so as to satisfy these three conditions. One might for instance insist that the team of engineers should all be of one sex, but this would not really be satisfactory, for it is probably possible to rear a complete individual from a single cell of the skin (say) of a man. To do so would be a feat of biological technique deserving of the very highest praise, but we would not be inclined to regard it as a case of “constructing a thinking machine.” This prompts us to abandon the requirement that every kind of technique should be permitted. We are the more ready to do so in view of the fact that the present interest in “thinking machines” has been aroused by a particular kind of machine, usually called an “electronic computer” or “digital computer.” Following this suggestion we only permit digital computers to take part in our game. . . .
This special property of digital computers, that they can mimic any discrete machine, is described by saying that they are universal machines. The existence of machines with this property has 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 tat as a consequence of this all digital computers are in a sense equivalent.


Contrary View on the Main Question

We may now consider the ground to have been cleared and we are ready to proceed to the debate on our question “Can machines think?”
... We cannot altogether abandon our original form of the problem, for opinions will differ as to the appropriateness of the substitution and we must at least listen to what has to be said in this connection.
It will simplify matters for the reader if I explain first my own beliefs in the matter. Consider first the more accurate form of the question. I

believe that in about fifty years time it will be possible to program computers, with a storage capacity of about 109, to make them play the imitation game so well that an average interrogator will not have more that 70 percent chance of making the right identification after five minutes of questioning. The original question, “Can machines think?” I believe to be too meaningless to deserve discussion. Nevertheless I believe that at the end of the century the use of words and general educated opinion will have altered so much that one will be able to speak of machines thinking without expecting to be contradicted. I believe further that no useful purpose is served by concealing these beliefs. The popular view is that scientists proceed inexorably from well-established fact to well-established fact, never being influenced by any unproved conjecture is quite mistaken. Provided it is made clear which are proved facts and which are conjectures, no harm can result. Conjectures are of great importance since they suggest useful lines of research.
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 machines. Hence no animal or machine can think.1
I am unable to accept any part of this, but will attempt to reply in theological terms. I should find this argument more convincing if animals were classed with men, for there is a greater difference, to my mind, between the typical animate than there is between man and the other animals. 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? But let us leave this point aside and return to the main argument. It appears to me that the argument quoted above implies a serious restriction to the omnipotence of the Almighty. It is admitted that there are certain things 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? We might expect that He would only exercise this power in conjunction with a mutation which provided the elephant with an appropriately improved brain to minister to the need of this soul. An argument of exactly similar form may be made for the case

1Possibly this view is heretical. St Thomas Aquinas (Summa Theologica, quoted by Bertrand Russell, A History of Western Philosophy (New York: Simon and Schuster, 1945), p. 458) states that God cannot make man to have no soul. But this may not be a real restriction on His powers, but only a result of the fact that men’s souls are immortal and therefore indestructible.

of machines. It may seem different because it is more difficult to “swallow.” But this really only means that we think it would be less likely that He would consider the circumstances suitable for conferring a soul. The circumstances in question are discussed in the rest of this paper. In attempting to construct such machines we should not be irreverently usurping His power of creating souls, any more tan we are in the procreation of children, rather, we are, in either case, instruments of His will providing mansions for the souls that He creates.
However, this is mere speculation. I am not very impressed with theological arguments whatever they may be used to support. Such arguments have often been found unsatisfactory in the past. In the time of Galileo it was argued that the texts, “And the sun stood still . . and hasted not to go down about a whole day” (Joshua x, 13) and “He laid the foundation of the earth, that it should not move at any time” (Psalm cv. 5) were an adequate refutation of the Copernican theory. With our present knowledge such an argument appears futile. When that knowledge was not available it made quite a different impression.
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.
This argument is seldom expressed quite so openly as in the form above. But it affects most of us who think about it at all. We like to believe that Man is in some subtle way superior to the rest of creation. It is best if he can be shown to be necessarily superior, for then there is no danger of him losing his commanding position. The popularity of the theological argument is clearly connected with this feeling. It is likely to be quite strong in intellectual people, since they value the power of thinking more highly than others, and are more inclined to base their belief in the superiority of Man on this power.
I do not think 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 results is known as Gödel’s theorem, and shows that in any sufficiently powerful logical system statements can be formulated which can neither be proved nor disproved within the system, unless possibly the system itself is inconsistent. There are other, in some respects similar, results due to Church, Kleene, Roser and Turing. The latter result is the most convenient to consider, since it refers directly to machines, whereas the others can only be used in a comparatively indirect argument: for instance if Gödel’s

theorem is to be used we need in addition to have some means of describing logical systems in terms of machines, and machines in terms of logical systems. The result in question refers to a type of machine which is essentially a digital computer with an infinite capacity. It states that there are certain things that such a machine cannot do. If it is rigged up to give answers to questions as in the imitation game, there will be some questions to which it will either give a wrong answer, or fail to give an answer at all however much time is allowed for a reply. There may, of course, be many such questions and questions which cannot be answered by one machine may be satisfactorily answered by another. We are of course supposing for the present that the questions are of the kind to which an answer “Yes” or “No” is appropriate, rather than questions such as “What do you think of Picasso?” The questions that we know the machines must fail on are of this type. “Consider the machine specified as follows . . . Will this machine ever answer “Yes” to any question?” The dots are to be replaced by a description of some machine in a standard form . . . When the machine described bears a certain comparatively simple relation to the machine which is under interrogation, it can be shown that the answer is either wrong or not forthcoming. This is the mathematical result: it is argued that it proves a disability of machines to which the human intellect is not subject.
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 only been stated, without any sort of proof, that no such limitations apply to the human intellect. But I do not think that this view can be dismissed quite so lightly. Whenever one of these machines is asked the appropriate critical question, and gives a definite answer, we now that this answer must be wrong, and this gives us a certain feeling of superiority. Is this feeling illusory? It is no doubt qite 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 with such evidence of fallibility on the part of the machines. Further, our superiority can only be felt on such a 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.
Those who hold to the mathematical argument would, I think, mostly be willing to accept the imitation game as a basis for discussion. Those who believe in the two previous objections would probably not be interested in any criteria.
4. The Argument from Consciousness. This argument is very well ex-

Pressed in Professor Jefferson’s Lister Oration for 1949, from which I quote. “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, be charmed by sex, be angry or depressed when it cannot get what it wants.”
This argument appears to be a denial of the validity of our test. According to the most extreme form of this view the only way by which one could be sure that a 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 what a man thinks is to be that particular man. It is in fact the solipsist point of view. It may be the most logical view to hold but it makes communication of ideas difficult. A is liable to believe “A thinks but B does not” while B believes “B thinks but A does not.” Instead of arguing continually over this point it is usual to have the polite convention that everyone thinks.
I am sure that Professor Jeffereon does not wish to adopt the extreme and solipsist point of view. Probably he would be quite willing to accept the imitation game as a test. The game (with the player B omitted) is frequently used in practice under the name of viva voce to discover whether someone really understands something or has “learned it parrot fashion.” Let us listen 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.
WITNES: Yes, but nobody wants to be compared to a winter’s day.
INTERROGATOR: Would you say that Mr. Pickwic 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 signaling” 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 contriv-

ance.” This phrase is, I think, intended to cover such devices as the inclusion in the machine of a record of someone reading a sonnet, with appropriate switching to turn it on from time to time.
In short, then, I think that most of those who support the argument from consciousness could be persuaded to abandon it rather than be forced into the solipsist position. They will then probably be willing to accept our test.
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 localize 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.” Numerous features X are suggested in this connection. I offer a selection.

Be kind, resourceful, beautiful, friendly . . . have initiative, have a sense of humour, tell right from wrong, make mistakes . . . fall in love, enjoy strawberries and cream . . . make someone fall in love with it, learn from experience . . . use word properly, be the subject of its own thought. . . have as much diversity of behaviour as a man, do something really new.

No support is usually offered for these statements. I believe they are mostly founded on the principle of scientific induction. A man has seen thousands of machines in his lifetime. From what he sees of them he draws a number of general conclusions. They are ugly, each is designed for a very limited purpose, when required for a minutely different purpose they are useless, the variety of behaviour of any one of them is very small, etc. Naturally he concludes that these limitations are associated with the very small storage capacity of most machines. (I am assuming that the idea of storage capacity is extended in some way to cover machines other than discrete state machines. The exact definition does not matter as no mathematical accuracy is claimed in the present discussion.) A few years ago, when very little had been heard of digital computers, it was possible to elicit much incredulity concerning them, if one mentioned their properties without describing their construction. That was presumably due to a similar application of the principle of of scientific induction. These applications o the principle are of course largely unconscious. When a burned child fears the fire and shows that he fears it by avoiding it, I should say he was applying scientific induction. (I could of course also describe his behaviour in many other ways.) The works and customs

of mankind do not seem to be very suitable material to which to apply scientific induction. A very large part of space-time must be investigated if reliable results are to be obtained. Otherwise we may (as most English children do) decide that everybody speaks English, and then it is silly to learn French.
There are, however, special remarks to be made about many of the disabilities that have been mentioned. 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, e.g., 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 the any the worse for that?” But let us adopt a more sympathetic attitude, and try to see what is really meant. I think this criticism can be explained in terms of the imitation game. It is claimed that the interrogator could distinguish the machine from the man simply by setting them a number of problems in arithmetic. The machine would be unmasked because of its deadly accuracy. The reply to that is simple. The machine (programmed for playing the game) would not attempt to give the right answers to the arithmetic questions. It would deliberately introduce mistakes in a manner calculated to confuse the interrogator. A mechanical fault would probably show itself through an unsuitable decision to what sort of mistake to make in the arithmetic. Even this interpretation of the criticism is not sufficiently sympathetic. But we cannot afford the space to go into it much further. It seems to me that this criticism depends on a confusion between two kinds of mistakes. We may call them “errors of functioning” and “errors of conclusion.” Errors of functioning are due to some mechanical or electrical fault which causes the machine to behave otherwise than it was designed to do. In philosophical discussions one likes to ignore the possibility of such errors, one is therefore discussing “abstract machines.” These abstract machines are mathematical fictions rather than physical objects.. By definition they are incapable of errors of functioning. In this sense we can truly say that “machines can never make mistakes. ”Errors of conclusion can only arise when some meaning is attached to the output signals from the machine. The machine might, for instance, type out mathematical equations, or sentences in English. When a false proposition is typed we say that the machine has committed an error of conclusion. There is clearly no reason at all for saying that a machine cannot make this kind of mistake. It might do nothing but type out repeatedly “0=1.” To take

A less perverse example, it might have some method for drawing conclusions by scientific induction. We must expect such a method to lead occasionally to erroneous results.
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. Nevertheless, “the subject matter of a machine’s operations” does seem to mean something, at least to the people who deal with it. If, for instance, the machine was trying to find a solution of the equation X2 -40x – 11 = 0, one would be tempted to describer this equation as part of the machine’s subject matter at that moment. In this sort of sense a machine undoubtedly can be its own subject matter. It may be used to help in making up its own programs, or to predict the effect of alterations in its own structure. By observing the results of its own behaviour it can modify its own programs so as to achieve some purpose more effectively. These are possibilities of the near future, rather than Utopian dreams.
The criticism that a machine cannot have much diversity of behaviour is just a way of saying that it cannot have much storage capacity. Until fairly recently a storage capacity of even a thousand digits was very rare.
The criticisms that we are considering here are often disguised forms of the argument from consciousness. Usually if one maintains that a machine can do one of these things and describes the kind of method that the machine could use, one will not make much of an impression. It is thought that the method (whatever it may be, for it must be mechanical) is really rather base. Compare the parenthesis in Jefferson’s statement quoted above.
6. Lady Lovelace’s objection. Our most detailed information of Babbage’s Analytical Engine comes from a memoir by Lady Lovelace. 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) This statement is quoted by Hartree who adds: “This does not imply that it may no be possible to construct electronic equipment which will think for itself,” or in which, in biological terms, one could set up a conditioned reflex, which would serve as a basis for ‘learning.’ Whether that is possible in principle or not is a a stimulating and exciting question, suggested by some of these recent developments. But it did not seem that the machines constructed or projected at the time had this property.”
I am in thorough agreement with Hartree over this. It will be noticed that he does not assert that the machines in question had not got the property, but rather that the evidence available to Lady Lovelace did not encourage her to believe that they had it. It is quite possible that the

machines in question had in a sense got this property. For suppose that some discrete state machine has the property. The Analytical Engine was a universal digital computer, so that, if its storage capacity and speed were adequate, it could by suitable programming be made to mimic the machine in question. Probably this argument did not occur to the Countess or to Babbage. In any case there was no obligation on them to claim all that could be claimed.
A variant of Lad Lovelace’s objection states that a machine can “never do anything really new.” This may be parried for a moment with the saw, “There is nothing new under the sun.” Who can be certain that “original work” that he has done was not simply the growth of the seed planted in him by teaching, or by the effect of following well-known general principles? A better variant of the objection says that a machine can never “take us by surprise.” This statement is a more direct challenge and can be met directly. Machines take me by surprise with great frequency. This is largely because I do not do sufficient calculation to decide what to expect them to do, or rather because, although I do a calculation, I do it in a hurried, slipshod fashion, taking risks. Perhaps I say to myself, “I suppose the voltage here ought to be the same as there; anyway let’s assume it is.” Naturally I am often wrong, and the result is a surprise for me, for by the time the experiment is done these assumptions have been forgotten. These admissions lay me open to lectures on the subject of my vicious ways, but do not throw any doubt on my credibility when I testify to the surprises I experience.
I do not expect this reply to silence my critic. He will probably say that such surprises are due to some creative mental act on my part, and reflect no credit on the machine. This leads us back to the argument from consciousness and far from the idea of surprise. It is a line of argument we must consider closed, but it is perhaps worth remembering that the appreciation of something as surprising requires as much of a “creative mental act” whether the surprising event originates from a man, a book, a machine or anything else.
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. A natural consequence of doing so is that one then assumes that there is no virtue in the mere working out of consequences from data and general principles.
7. Argument from Continuity in the Nervous System. The nervous system

is certainly not a discrete state machine. A small error in the information about the size of a nervous impulse impinging on a neuron may make a large different ce to the size of the outgoing impulse. It may be argued that, this being so, one cannot expect to be able to mimic the behaviour of the nervous system with a discrete state system.
It is true that a discrete state machine must be different from a continuous machine. But if we adhere to the condition of the imitation game, the interrogator will not be able to take any advantage of this difference. The situation can be made clearer if we consider some other simpler continuous machine. (A differential analyzer is a certain kind of machine not of the discrete state type used for some kinds of calculation.) Some of these provide their answers in a type form, and so are suitable for taking part in the game. It would not be possible for a digital computer to predict exactly what answers the differential analyzer would give to a problem, but it would be quite capable of giving the right sort of answer. For instance if asked to give the value of π (actually about 3.1416) it would be reasonable to choose at random between the values 3.12, 3.13, 3.14, 3.15, 3.16 with the probabilities of 0.05, 0.15, 0.55, 0.19, 0.06 (say). Under these circumstances it would be very difficult for the interrogator to distinguish the differential analyzer from the digital computer.
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 perhaps decide that it is safest to stop. But some further difficulty may well arise from this decision later. To attempt to provide rules of conduct to cover every eventuality, even those arising from traffic lights, appears to be impossible. With all this I agree.
From this it is argued that we cannot be machines. I shall try to reproduce the argument, but I fear I shall hardly do it justice. It seems to run something like this. “If each man had a definite set of rules of conduct by which he regulated his life he would be no better than a machine. But there are no such rules, so men cannot be machines.” The undistributed middle is quite glaring. I do not think the argument is ever put quite like this, but I believe this is the argument used nevertheless. There may however be certain confusion between “rules of conduct” and “laws of behaviour” to cloud the issue. BY rules of conduct I mean precepts such as “Stop if you see red lights,“ on which one can act, and of which one can be conscious. By “laws of behaviour” I mean laws of

nature as applied to a man’s body such as “if you pinch him he will squeak.” If we substitute “laws of behaviour which regulate his life” for “laws of conduct by which he regulates his life” in the argument quoted the undistributed middle is no longer insuperable. For we believe that it is not only true that being regulated by laws of behaviour implies being some sort of machine (though not necessarily a discrete state machine), but that conversely being such a machine implies being regulated by such laws. However, we cannot do easily convince ourselves of the absence of complete laws of behaviour as complete rules of conduct. The only way we know of for finding such laws is scientific observation, and we certainly know of no circumstances under which we could say, “We have searched enough. There are no such laws.”
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 with a reasonable time, say a thousand years. But this does not seem to be the case. I have set up on the Manchester computer a small program using only 1000 units of storage, whereby the machine supplied with one sixteen-figure number replies with another within two seconds. I would defy anyone to learn from these replies sufficient about the program to be able to predict any replies to untried values.
9. The Argument from Extrasensory Perception. I assume the reader is familiar with the idea of extrasensory perception, and the meaning of the four items of it, viz., 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. It is very difficult to rearrange one’s ideas so as to fit these new facts in. Once one has accepted them it does not seem a very big step to believe in ghosts and bogies. The idea that our bodies move simply according to the known laws of physics, together with some others not yet discovered but somewhat similar, would be one of the first to go.
His argument is to my mind quite a strong one, One can say in reply that many scientists theories seem to remain workable in practice, in spite of clashing with E.S.P.; but in fact one can get along very nicely if one forgets about it. This is rather cold comfort, and one fears that thinking is just the kind of phenomenon where E.S.P. may be especially relevant.
A more specific argument based on E.S.P. might run as follows: “Let us play the imitation game, using as witnesses a man who is good as a telepathic receiver, and a digital computer. The interrogator can ask such questions as ‘What suit does the card in my right hand belong to?’ The

Man by telepathy or clairvoyance gives the right answer 130 times out of 400 cards. The machine can only guess at random and perhaps gets 104 right, so the interrogator makes the right identification.” There is an interesting possibility, which opens here. Suppose the digital computer contains a random number generator. Then it will be natural to use this to decide what answer to give. But then the random number generator will be subject to the psychokinetic powers of the interrogator. Perhaps this pschokinesis might cause the machine to guess right more often than would be expected on a probability calculation, so that te interrogator might still be unable to make the right identification. On the other hand, he might be able to guess right without any questioning, by clairvoyance. With E.S.P. anything may happen.
If telepathy is admitted it will be necessary to tighten our test. The situation could be regarded as analogous to that which would occur if the interrogator were talking to himself and one of the competitors was listening with his ear to the wall. To put the competitors into a “telepathy-proof room” would satisfy all requirements.


A.M. TURING


Reflections


Most of our response to this remarkable and lucid article is contained in the following dialogue. However, we wish to make a short comment about Turing’s apparent willingness to believe that extrasensory perception might turn out to be the ultimate difference between humans and the machines they create. If this comment is taken at face value (and not as some sort of discrete joke), one has to wonder what motivated it. Apparently Turing was convinced that the evidence for telepathy was quite strong. However, if it was strong in 1950, it is no stronger now, thirty years later – in fact, it is probably weaker. Since 1950 there have been many notorious cases of claims of psychic ability of one sort or another, often vouched for by physicists of some renown. Some of these physicists have later felt they had been made fools of and have taken back their public pro-E.S.P. pronouncements, only to jump on some new paranormal bandwagon the next month. But it is safe to say that the majority of physicists – and certainly the majority of psychologists, who specialize in understanding the mind – doubt the existence of extrasensory perception in any form.

Turing took “cold comfort” in the idea that paranormal phenomena might be reconcilable in some way with well-established scientific theories. We differ with him. We suspect hat if such phenomena such as telepathy, precognition, and telekinesis turned out to exist (and turned out to have he remarkable properties typically claimed for them).the laws of physics would not be simply amend able to accommodate them; only a major revolution in our scientific world view could do them justice. One might look forward to such a revolution with sadness and perplexity. How could the science that had worked for so well for so many things turn out to be so wrong? The challenge of rethinking al of science from its most basic assumptions on up would be a great intellectual adventure, but the evidence that we need to do this has simply failed to accumulate over the years.

D.R.H.
D.C.D.

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