[Amin_Mansouri]

Turing, A.M. (1950). Computing machinery and intelligence. Mind, 59,
433-460.

[quote]
[quote]

COMPUTING MACHINERY AND
INTELLIGENCE

By A. M. Turing

1. The Imitation Game


I propose to consider the question, "Can machines think?" This should begin
with definitions of the meaning of the terms "machine" and "think." The
definitions might be framed so as to reflect so 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 and the answer to the
question, "Can machines think?" is to be sought in a statistical survey such as
a Gallup 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 © who may be of either s /\/\///\e///:/x. 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. 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 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 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 for the third
player (B) is to help the interrogator. The best strategy for her 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
remarks.


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?
These questions replace our original, "Can machines think?"


2. 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
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 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 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 penalise the machine for its inability to shine in beauty competitions, nor
to penalise a man for losing in a race against an aeroplane. 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 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? This objection 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.


3. The Machines Concerned in the Game


The question which we put in 1 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 the possibility than 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 be all of one s /\/\///\e///:/x, 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 restriction appears at first sight to be a very drastic one. I shall
attempt to show that it is not so in reality. To do this necessitates a short
account of the nature and properties of these computers.


It may also be said that this identification of machines with digital
computers, like our criterion for "thinking," will only be unsatisfactory if
(contrary to my belief), it turns out that digital computers are unable to give
a good showing 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? It would be easy to satisfy
the conditions of the game. A number of interrogators could be used, and
statistics compiled to show how often the right identification was given." The
short answer is that we are not asking whether all digital computers would do
well in the game nor whether the computers at present available would do well,
but whether there are imaginable computers which would do well. But this is only
the short answer. We shall see this question in a different light later.


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. He may
also do his multiplications and additions on a "desk machine," but this is not
important.

If we use the above explanation as a definition we shall be in danger of
circularity of argument. We avoid this by giving an outline. of the means by
which the desired effect is achieved. 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. In so far as the human computer does
calculations in his bead a part of the store will correspond to his
memory.


The executive unit is the part which carries out the various individual
operations involved in a calculation. What these individual operations are will
vary from machine to machine. Usually fairly lengthy operations can be done such
as "Multiply 3540675445 by 7076345687" but in some machines only very simple
ones such as "Write down 0" are possible.


We have mentioned that 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 control is so constructed that this
necessarily happens.


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. In this case
the)e operation is that described above, viz., "Add the number. . . ." It will
be noticed that the instruction takes up 10 digits and so forms one packet of
information, very conveniently. 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"


may be encountered, or again


"If position 4505 contains 0 obey next the instruction stored in 6707,
otherwise continue straight on."


Instructions of these latter types 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, but in doing so to obey, not fresh instructions on
each repetition, but the same ones over and over again. 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 if he has the shoes
with him.


The reader must accept it as a fact that digital computers can be
constructed, and indeed have been constructed, according to the principles we
have described, and that they can in fact mimic the actions of a human computer
very closely.


The book of rules which we have described our human computer as using is of
course a convenient fiction. Actual human computers really remember what they
have got to do. 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." To "programme a
machine to carry out the operation A" means to put the appropriate instruction
table into the machine so that it will do A.


An interesting variant on the idea of a digital computer is a "digital
computer with a random element." These have instructions involving the throwing
of a die or some equivalent electronic process; one such instruction might for
instance be, "Throw the die and put the-resulting number into store 1000."
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
.


Most actual digital computers have only a finite store. There is no
theoretical difficulty in the idea of a computer with an unlimited store. Of
course only a finite part can have been used at any one time. Likewise only a
finite amount can have been constructed, but we can imagine more and more being
added as required. 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, Lucasian
Professor of Mathematics at Cambridge from 1828 to 1839, planned such a machine,
called the Analytical Engine, but it was never completed. Although Babbage had
all the essential ideas, his machine was not at that time such a very attractive
prospect. The speed which would have been available would be definitely faster
than a human computer but something like I 00 times slower than the Manchester
machine, itself one of the slower of the modern machines, The storage was to be
purely mechanical, using wheels and cards.


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. Of course electricity usually comes in
where fast signalling is concerned, so that it is not surprising that we find it
in both these connections. In the nervous system chemical phenomena are at least
as important as electrical. In certain computers the storage system is mainly
acoustic. The feature of using electricity is thus seen to be only a very
superficial similarity. If we wish to find such similarities we should took
rather for mathematical analogies of function.


5. Universality of Digital Computers


The digital computers considered in the last section 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. These states
are sufficiently different for the possibility of confusion between them to be
ignored. Strictly speaking there, are no such machines. Everything really moves
continuously. But there are many kinds of machine which can profitably be
thought of as being discrete-state machines. For instance in considering the
switches for a lighting system it is a convenient fiction that each switch must
be definitely on or definitely off. There must be intermediate positions, but
for most purposes we can forget about them. As an example of a discrete-state
machine we might consider a wheel which clicks round through 120 once a second,
but may be stopped by a ]ever which can be operated from outside; in addition a
lamp is to light in one of the positions of the wheel. This machine could be
described abstractly as follows. The internal state of the machine (which is
described by the position of the wheel) may be q1, q2 or
q3. There is an input signal i0. or i1 (position of ]ever). The
internal state at any moment is determined by the last state and input signal
according to the table

(TABLE DELETED)

The output signals, the only externally visible indication of the
internal state (the light) are described by the table


State q1 q2 q3

output o0 o0 o1


This example is typical of discrete-state machines. They can be described by
such tables provided they have only a finite number of possible states.


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 prediction which we are
considering is, however, rather nearer to practicability than that considered by
Laplace. The system of the "universe as a whole" is such that quite small errors
in the initial conditions can have an overwhelming effect at a later time. 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.


As we have mentioned, 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 2 165,000, i.e., about 10
50,000. Compare this with our example of the clicking wheel described
above, which had three states. It is not difficult to see why the number of
states should be so immense. The computer includes a store corresponding to the
paper used by a human computer. It must be possible to write into the store any
one of the combinations of symbols which might have been written on the paper.
For simplicity suppose that only digits from 0 to 9 are used as symbols.
Variations in handwriting are ignored. Suppose the computer is allowed 100
sheets of paper each containing 50 lines each with room for 30 digits. Then the
number of states is 10 100x50x30 i.e., 10 150,000 . This
is about the number of states of three Manchester machines put together. The
logarithm to the base two of the number of states is usually called the "storage
capacity" of the machine. Thus the Manchester machine has a storage capacity of
about 165,000 and the wheel machine of our example about 1.6. If two machines
are put together their capacities must be added to obtain the capacity of the
resultant machine. This leads to the possibility of statements such as "The
Manchester machine contains 64 magnetic tracks each with a capacity of 2560,
eight electronic tubes with a capacity of 1280. Miscellaneous storage amounts to
about 300 making a total of 174,380."


Given the table corresponding to a discrete-state machine it is possible to
predict what it will do. There is no reason why this calculation should not be
carried out by means of a digital computer. Provided it could be carried out
sufficiently quickly the digital computer could mimic the behavior of any
discrete-state machine. The imitation game could then be played with the machine
in question (as B) and the mimicking digital computer (as A) and the
interrogator would be unable to distinguish them. Of course the digital computer
must have an adequate storage capacity as well as working sufficiently fast.
Moreover, it must be programmed afresh for each new machine which it is desired
to mimic.


This special property of digital computers, that they can mimic any
discrete-state 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 'ill be seen that as a
consequence of this all digital computers are in a sense equivalent.


We may now consider again the point raised at the end of §3. It was suggested
tentatively that the question, "Can machines think?" should be replaced by "Are
there imaginable digital computers which would do well in the imitation game?"
If we wish we can make this superficially more general and ask "Are there
discrete-state machines which would do well?" But in view of the universality
property we see that either of these questions is equivalent to this, "Let us
fix our attention on one particular digital computer C. Is it true that by
modifying this computer to have an adequate storage, suitably increasing its
speed of action, and providing it with an appropriate programme, C can be made
to play satisfactorily the part of A in the imitation game, the part of B being
taken by a man?"


6. Contrary Views 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?" and the variant of
it quoted at the end of the last section. We cannot altogether abandon the
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
connexion.


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 programme 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 than 70 per cent 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 that scientists proceed inexorably from well-established fact
to well-established fact, never being influenced by any improved 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 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. I should find the argument more convincing if animals were
classed with men, for there is a greater difference, to my mind, between the
typical animate and the inanimate 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 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? 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 needs of this sort[. An
argument of exactly similar form may be made for the case 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 than
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 foundations 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 a quite 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 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 results 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, unless possibly the system
itself is inconsistent. There are other, in some respects similar, results due
to Church (1936), Kleene (1935), Rosser, and Turing (1937). 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 Godel'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, which could be something like that used in §5. 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 If 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 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 know that this answer must be wrong, and this gives
us a certain feeling of superiority. Is this feeling illusory? It 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.

Those who hold to the mathematical argument would, I think, mostly he 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 expressed 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 s /\/\///\e///:/x, 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 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. 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" whilst 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 Jefferson 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 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." 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 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."
Numerous features X are suggested in this connexion 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 some one fall in love with it, learn from experience, use words
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., etc. Naturally he concludes
that these are necessary properties of machines in general. Many of 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 scientific
induction. These applications of the principle are of course largely
unconscious. When a burnt child fears the fire and shows that he fears it by
avoiding it, f should say that 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 that 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 they 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 this is simple. The machine (programmed for
playing the game) would not attempt to give the right answers to the arithmetic
problems. It would deliberately introduce mistakes in a manner calculated to
confuse the interrogator. A mechanical fault would probably show itself through
an unsuitable decision as to what sort of a 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 mistake, 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 "O = I." 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 describe 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 programmes, or to predict the effect of alterations in its own
structure. By observing the results of its own behaviour it can modify its own
programmes 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 tile method
(whatever it may be, for it must be mechanical) is really rather base. Compare
the parentheses in Jefferson's statement quoted on page 22.


(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). This statement is quoted by Hartree (
1949) who adds: "This does not imply that it may not 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 this is possible in principle or not is 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.


This whole question will be considered again under the heading of learning
machines.


A variant of Lady 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 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 he 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 h
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 remarking 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 difference 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 conditions 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 sonic other simpler continuous
machine. A differential analyser will do very well. (A differential analyser 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 typed 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 analyser 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 (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 analyser 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 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 a 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 so easily convince
ourselves of the absence of complete laws of behaviour as of 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
within 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 programme using only
1,000 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 programme to be able to predict any
replies to untried values.


(9) The Argument from Extrasensory Perception


I assume that 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.


This argument is to my mind quite a strong one. One can say in reply that
many scientific theories seem to remain workable in practice, in spite of
clashing with ESP; that 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 ESP may be especially relevant.


A more specific argument based on ESP 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
psychokinesis might cause the machine to guess right more often than would be
expected on a probability calculation, so that the 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 ESP anything may
happen.


If telepathy is admitted it will be necessary to tighten our test up. 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.


7. Learning Machines


The reader will have anticipated that I have no very convincing arguments of
a positive nature to support my views. If I had I should not have taken such
pains to point out the fallacies in contrary views. Such evidence as I have I
shall now give.


Let us return for a moment to Lady Lovelace's objection, which stated that
the machine can only do what we tell it to do. One could say that a man can
"inject" an idea into the machine, and that it will respond to a certain extent
and then drop into quiescence, like a piano string struck by a hammer. Another
simile would be an atomic pile of less than critical size: an injected idea is
to correspond to a neutron entering the pile from without. Each such neutron
will cause a certain disturbance which eventually dies away. If, however, the
size of the pile is sufficiently increased, tire disturbance caused by such an
incoming neutron will very likely go on and on increasing until the whole pile
is destroyed. Is there a corresponding phenomenon for minds, and is there one
for machines? There does seem to be one for the human mind. The majority of them
seem to be "subcritical," i.e., to correspond in this analogy to piles of
subcritical size. An idea presented to such a mind will on average give rise to
less than one idea in reply. A smallish proportion are supercritical. An idea
presented to such a mind that may give rise to a whole "theory" consisting of
secondary, tertiary and more remote ideas. Animals minds seem to be very
definitely subcritical. Adhering to this analogy we ask, "Can a machine be made
to be supercritical?"


The "skin-of-an-onion" analogy is also helpful. In considering the functions
of the mind or the brain we find certain operations which we can explain in
purely mechanical terms. This we say does not correspond to the real mind: it is
a sort of skin which we must strip off if we are to find the real mind. But then
in what remains we find a further skin to be stripped off, and so on. Proceeding
in this way do we ever come to the "real" mind, or do we eventually come to the
skin which has nothing in it? In the latter case the whole mind is mechanical.
(It would not be a discrete-state machine however. We have discussed

this.)


These last two paragraphs do not claim to be convincing arguments. They
should rather be described as "recitations tending to produce belief."


The only really satisfactory support that can be given for the view expressed
at the beginning of §6, will be that provided by waiting for the end of the
century and then doing the experiment described. But what can we say in the
meantime? What steps should be taken now if the experiment is to be successful?



As I have explained, the problem is mainly one of programming. Advances in
engineering will have to be made too, but it seems unlikely that these will not
be adequate for the requirements. Estimates of the storage capacity of the brain
vary from 1010 to 1015 binary digits. I incline to the
lower values and believe that only a very small fraction is used for the higher
types of thinking. Most of it is probably used for the retention of visual
impressions, I should be surprised if more than 109 was required for
satisfactory playing of the imitation game, at any rate against a blind man.
(Note: The capacity of the Encyclopaedia Britannica, 11th edition, is 2 X
109) A storage capacity of 107, would be a very
practicable possibility even by present techniques. It is probably not necessary
to increase the speed of operations of the machines at all. Parts of modern
machines which can be regarded as analogs of nerve cells work about a thousand
times faster than the latter. This should provide a "margin of safety" which
could cover losses of speed arising in many ways, Our problem then is to find
out how to programme these machines to play the game. At my present rate of
working I produce about a thousand digits of progratiirne a day, so that about
sixty workers, working steadily through the fifty years might accomplish the
job, if nothing went into the wastepaper basket. Some more expeditious method
seems desirable.


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,


© 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? If this were then
subjected to an appropriate course of education one would obtain the adult
brain. Presumably the child brain is something like a notebook as one buys it
from the stationer's. Rather little mechanism, and lots of blank sheets.
(Mechanism and writing are from our point of view almost synonymous.) Our hope
is that there is so little mechanism in the child brain that something like it
can be easily programmed. The amount of work in the education we can assume, as
a first approximation, to be much the same as for the human child.


We have thus divided our problem into two parts. The child programme and the
education process. These two remain very closely connected. We cannot expect to
find a good child machine at the first attempt. One must experiment with
teaching one such machine and see how well it learns. One can then try another
and see if it is better or worse. There is an obvious connection between this
process and evolution, by the identifications


Structure of the child machine = hereditary material


Changes of the child machine = mutation,


Natural selection = judgment of the experimenter


One may hope, however, that this process will be more expeditious than
evolution. The survival of the fittest is a slow method for measuring
advantages. The experimenter, by the exercise of intelligence, should he able to
speed it up. Equally important is the fact that he is not restricted to random
mutations. If he can trace a cause for some weakness he can probably think of
the kind of mutation which will improve it.


It will not be possible to apply exactly the same teaching process to the
machine as to a normal child. It will not, for instance, be provided with legs,
so that it could not be asked to go out and fill the coal scuttle. Possibly it
might not have eyes. But however well these deficiencies might be overcome by
clever engineering, one could not send the creature to school without the other
children making excessive fun of it. It must be given some tuition. We need not
be too concerned about the legs, eyes, etc. The example of Miss Helen Keller
shows that education can take place provided that communication in both
directions between teacher and pupil can take place by some means or other.



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 machine has to be so constructed that events which shortly
preceded the occurrence of a punishment signal are unlikely to be repeated,
whereas a reward signal increased the probability of repetition of the events
which led up to it. These definitions do not presuppose any feelings on the part
of the machine, I have done some experiments with one such child machine, and
succeeded in teaching it a few things, but the teaching method was too
unorthodox for the experiment to be considered really successful.


The use of punishments and rewards can at best be a part of the teaching
process. Roughly speaking, if the teacher has no other means of communicating to
the pupil, the amount of information which can reach him does not exceed the
total number of rewards and punishments applied. By the time a child has learnt
to repeat "Casabianca" he would probably feel very sore indeed, if the text
could only be discovered by a "Twenty Questions" technique, every "NO" taking
the form of a blow. It is necessary therefore to have some other "unemotional"
channels of communication. If these are available it is possible to teach a
machine by punishments and rewards to obey orders given in some language, e.g.,
a symbolic language. These orders are to be transmitted through the
"unemotional" channels. The use of this language will diminish greatly the
number of punishments and rewards required.


Opinions may vary as to the complexity which is suitable in the child
machine. One might try to make it as simple as possible consistently with the
general principles. Alternatively one might have a complete system of logical
inference "built in."' In the latter case the store would be largely occupied
with definitions and propositions. The propositions would have various kinds of
status, e.g., well-established facts, conjectures, mathematically proved
theorems, statements given by an authority, expressions having the logical form
of proposition but not belief-value. Certain propositions may be described as
"imperatives." The machine should be so constructed that as soon as an
imperative is classed as "well established" the appropriate action automatically
takes place. To illustrate this, suppose the teacher says to the machine, "Do
your homework now." This may cause "Teacher says 'Do your homework now' " to be
included amongst the well-established facts. Another such fact might be,
"Everything that teacher says is true." Combining these may eventually lead to
the imperative, "Do your homework now," being included amongst the
well-established facts, and this, by the construction of the machine, will mean
that the homework actually gets started, but the effect is very satisfactory.
The processes of inference used by the machine need not be such as would satisfy
the most exacting logicians. There might for instance be no hierarchy of types.
But this need not mean that type fallacies will occur, any more than we are
bound to fall over unfenced cliffs. Suitable imperatives (expressed within the
systems, not forming part of the rules of the system) such as "Do not use a
class unless it is a subclass of one which has been mentioned by teacher" can
have a similar effect to "Do not go too near the edge."


The imperatives that can be obeyed by a machine that has no limbs are bound
to be of a rather intellectual character, as in the example (doing homework)
given above. important amongst such imperatives will be ones which regulate the
order in which the rules of the logical system concerned are to be applied, For
at each stage when one is using a logical system, there is a very large number
of alternative steps, any of which one is permitted to apply, so far as
obedience to the rules of the logical system is concern