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01/07/2006 05:22 PMCOMPUTING MACHINERY AND INTELLIGENCE
Page 1 of 22http://cogprints.org/499/00/turing.html
A. M. Turing (1950) Computing Machinery and Intelligence. Mind 49: 433-460.
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 (C) who may be of either sex. 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?"
01/07/2006 05:22 PMCOMPUTING MACHINERY AND INTELLIGENCE
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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.
01/07/2006 05:22 PMCOMPUTING MACHINERY AND INTELLIGENCE
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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 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 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
01/07/2006 05:22 PMCOMPUTING MACHINERY AND INTELLIGENCE
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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
01/07/2006 05:22 PMCOMPUTING MACHINERY AND INTELLIGENCE
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"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
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