Goedel's theorem seems to me to prove that Mechanism is
false, that is, that minds cannot be explained as
machines. So also has it seemed to many other people:
almost every mathematical logician I have put the matter
to has confessed to similar thoughts, but has felt
reluctant to commit himself definitely until he could see
the whole argument set out, with all objections fully
stated and properly met.^{1}
This I attempt to do.

Goedel's theorem states that in any consistent system which is strong enough to produce simple arithmetic there are formulae which cannot {44} be proved-in-the-system, but which we can see to be true. Essentially, we consider the formula which says, in effect, "This formula is unprovable-in-the-system". If this formula were provable-in-the-system, we should have a contradiction: for if it were provablein-the-system, then it would not be unprovable-in-the-system, so that "This formula is unprovable-in-the-system" would be false: equally, if it were provable-in-the-system, then it would not be false, but would be true, since in any consistent system nothing false can be provedin-the-system, but only truths. So the formula "This formula is unprovable-in-the-system" is not provable-in-the-system, but unprovablein-the-system. Further, if the formula "This formula is unprovablein- the-system" is unprovable-in-the-system, then it is true that that [256] formula is unprovable-in-the-system, that is, "This formula is unprovable-in-the-system" is true.

The foregoing argument is very fiddling, and difficult to grasp fully: it is helpful to put the argument the other way round, consider the possibility that "This formula is unprovable-in-the-system" might be false, show that that is impossible, and thus that the formula is true; whence it follows that it is unprovable. Even so, the argument remains persistently unconvincing: we feel that there must be a catch in it somewhere. The whole labour of Goedel's theorem is to show that there is no catch anywhere, and that the result can (113) be established by the most rigorous deduction; it holds for all formal systems which are (i) consistent, (ii) adequate for simple arithmetic---i.e., contain the natural numbers and the operations of addition and multiplication---and it shows that they are incomplete--- i.e., contain unprovable, though perfectly meaningful, formulae, some of which, moreover, we, standing outside the system, can see to be true.

Goedel's theorem must apply to cybernetical machines, because it is of the essence of being a machine, that it should be a concrete instantiation of a formal system. It follows that given any machine which is consistent and capable of doing simple arithmetic, there is a formula which it is incapable of producing as being true---i.e., the formula is unprovable-in-the-system-but which we can see to be true. It follows that no machine can be a complete or adequate model of the mind, that minds are essentially different from machines.

We understand by a cybernetical machine an apparatus
which performs a set of operations according to a
definite set of rules. Normally we "programme" a
machine: that is, we give it a set of instructions about
what it is to do in each eventuality; and we feed in the
initial "information" on which the machine is to perform
its calculations. When we {45} consider the possibility
that the mind might be a cybernetical mechanism we have
such a model in view; we suppose that the brain is
composed of complicated neural circuits, and that the
information fed in by the senses is "processed" and acted
upon or stored for future use. If it is such a
mechanism, then given the way in which it is
programmed---the way
in which it is "wired up"---and the information
which has been fed into it, the response---the
"output"---is determined, and could,
granted sufficient time, be
calculated. Our idea of a machine is just this, that its
behaviour is completely determined by the way it is made
and the incoming "stimuli": there is no possibility of
its acting on its own: given a certain form of
construction and a certain input of information, then it
must act in a certain specific way. We, however, shall
be concerned not with what a machine *must* do, but
with what it *can* do. That is, instead [257] of
considering the whole set of rules which together
determine exactly what a machine will do in given
circumstances, we shall consider only an outline of those
rules, which will delimit the possible responses of the
machine, but not completely. The complete rules will
determine the operations completely at every stage; at
every stage there will be a definite instruction, e.g.,
"If the number is prime and greater than two add one and
divide by two: if it is not prime, divide by its smallest
factor": we, however, will consider the possibility of
there being alternative instructions, e.g., "In a
fraction you may divide top and bottom by *any*
number which is a factor of both numerator and
denominator". In thus (114) relaxing the specification
of our model, so that it is no longer completely
determinist, though still entirely mechanistic, we shall
be able to take into account a feature often proposed for
mechanical models of the mind, namely that they should
contain a randomizing device. One could build a machine
where the choice between a number of alternatives was
settled by, say, the number of radium atoms to have
disintegrated in a given container in the past half-
minute. It is *prima facie* plausible that our
brains should be liable to random effects: a cosmic ray
might well be enough to trigger off a neural impulse.
But clearly in a machine a randomizing device could not
be introduced to choose any alternative whatsoever: it
can only be permitted to choose between a number of
allowable alternatives. It is all right to add any
number chosen at random to both sides of an equation, but
not to add one number to one side and another to the
other. It is all right to choose to prove one theorem of
Euclid rather than another, or to use one method rather
than another, but not to "prove" something which is not
true, or to use a "method of proof" which is not valid.
Any {46} randomizing devices must allow choices only
between those operations which will not lead to
inconsistency: which is exactly what the relaxed
specification of our model specifies Indeed, one might
put it this way: instead of considering what a completely
determined machine *must* do, we shall consider what
a machine might be able to do if it had a randomizing
device that acted whenever there were two or more
operations possible, none of which could lead to
inconsistency.

If such a machine were built to produce theorems about
arithmetic (in many ways the simplest part of
mathematics), it would have only a finite number of
components, and so there would be only a finite number of
types of operation it could do, and only a finite number
of initial (115) assumptions it could operate on.
Indeed, we can go further, and say that there would only
be a *definite* number of types of operation, and of
initial assumptions, that could be built into it.
Machines are definite: anything which was indefinite or
infinite we [258] should not count as a machine. Note
that we say number of types of operation, not number of
operations. Given sufficient time, and provided that it
did not wear out, a machine could go on repeating an
operation indefinitely: it is merely that there can be
only a definite number of different sorts of operation it
can perform.

If there are only a definite number of types of
operation and initial assumptions built into the system,
we can represent them all by suitable symbols written
down on paper. We can parallel the operation by rules
("rules of inference" or "axiom schemata") allowing us to
go from one or more formulae (or even from no formula at
all) to another formula, and we can parallel the initial
assumptions (if any) by a set of initial formulae
("primitive propositions", "postulates" or "axioms").
Once we have represented these on paper, we can represent
every single operation: all we need do is to give
formulae representing the situation before and after the
operation, and note which rule is being invoked. We can
thus represent on paper any possible sequence of
operations the machine might perform. However long, the
machine went on operating, we could, give enough time,
paper and patience, write down an analogue of the
machine's operations. This analogue would in fact be a
formal proof: every operation of the machine is
represented by the application of one of the rules: and
the conditions which determine for the machine whether an
operation can be performed in a certain situation,
become, in our representation, conditions which settle
whether a rule can be applied to a certain formula, i.e.,
formal conditions of applicability. Thus, construing our
rules as rules of inference, we shall have a proof-sequence
of {47} formulae, each one being written down in
virtue of some formal rule of inference having been
applied to some previous formula or formulae (except, of
course, for the initial formulae, which are given because
they represent initial assumptions built into the
system). The conclusions it is possible for the machine
to produce as being true will therefore correspond to the
theorems that can be proved in the corresponding formal
system. We now construct a Goedelian formula in this
formal system. This formula cannot be *proved-in-the-
system*. Therefore the machine cannot produce the
corresponding formula as being true. But we can see that
the Goedelian formula is true: any rational being could
follow Goedel's argument, and convince himself that the
Goedelian formula, although unprovable-in-the-system, was
nonetheless----in fact, for that very reason---true. Now
any mechanical model of the mind must include a mechanism
which can enunciate truths of arithmetic, because this is
something which minds can do: in fact, it is easy to
produce mechanical models which will in many respects
produce truths of arithmetic far [259] better than human
beings can. But in this one respect they cannot do so
well: in that for every machine there is a truth which it
cannot produce as being true, but which a mind can. This
shows that a machine cannot be a complete and adequate
model of the mind. It cannot do *everything* that a
mind can do, since however much it can do, there is
always something which it cannot do, and a mind can.
This is not to say that we cannot build a machine to
simulate any desired piece of mind-like behaviour: it is
only that we cannot build a machine to simulate
*every* piece of mind-like behaviour. We can (or
shall be able to one day) build machines capable of
reproducing bits of mind-like behaviour, and indeed of
outdoing the performances of human minds: but however
good the machine is, and however much better (116) it can
do in nearly all respects than a human mind can, it
always has this one weakness, this one thing which it
cannot do, whereas a mind can. The Goedelian formula is
the Achilles' heel of the cybernetical machine. And
therefore we cannot hope ever to produce a machine that
will be able to do all that a mind can do: we can never
not even in principle, have a mechanical model of the
mind.

This conclusion will be highly suspect to some people.
They will object first that we cannot have it both that a
machine *can* simulate *any* piece of mind-like
behaviour, and that it *cannot* simulate
*every* piece. To some it is a contradiction: to
them it is enough to point out that there is no
contradiction between the fact that for any natural
number there can be produced a greater number, and the
fact that a number cannot {48} be produced greater than
every number. We can use the same analogy also against
those who, finding a formula their first machine cannot
produce as being true, concede that that machine is
indeed inadequate, but thereupon seek to construct a
second, more adequate, machine, in which the formula can
be produced as being true. This they can indeed do: but
then the second machine will have a Goedelian formula all
of its own, constructed by applying Goedel's procedure to
the formal system which represents its (the second
machine's) own, enlarged, scheme of operations. And this
formula the second machine will not be able to produce as
being true, while a mind will be able to see that it is
true. And if now a third machine is constructed, able to
do what the second machine was unable to do, exactly the
same will happen: there will be yet a third formula, the
Goedelian formula for the formal system corresponding to
the third machine's scheme of operations, which the third
machine is unable to produce as being true, while a mind
will still be able to see that it is true. And so it
will go on. However complicated a machine we construct,
it will, if it is a machine, correspond to a formal
system, which in turn will be liable to the Goedel
procedure [260] for finding a formula unprovable-in-that-
system. This formula the machine will be unable to
produce as being true, although a mind can see that it is
true. And so the machine will still not be an adequate
model of the mind. We are trying to produce a model of
the mind which is mechanical---which is essentially
"dead"---but the mind, being in fact "alive", can always
go one better than any formal, ossified, dead, system
can. Thanks to Goedel's theorem, the mind always has the
last word.

A second objection will now be made. The procedure
whereby the Goedelian formula is constructed is a standard
procedure---only so could we be sure that a Goedelian
formula can be constructed for every formal system. But
if it is a standard procedure, then a machine should be
able to be programmed to carry it out too. We could
construct a machine with the usual operations, and in
addition an (117) operation of going through the Goedel
procedure, and then producing the conclusion of that
procedure as being true; and then repeating the
procedure, and so on, as often as required. This would
correspond to having a system with an additional rule of
inference which allowed one to add, as a theorem, the
Goedelian formula of the rest of the formal system, and
then the Goedelian formula of this new, strengthened
formal system, and so on. It would be tantamount to
adding. to the original formal system an infinite
sequence of axioms, each the Goedelian formula of the
system hitherto obtained. Yet even so, the matter is not
settled: for the machine with a Goedelizing {49} operator,
as we might call it, is a *different* machine from
the machines without such an operator; and, although the
machine with the operator would be able to do those
things in which the machines without the operator were
outclassed by a mind, yet we might expect a mind, faced
with a machine that possessed a Goedelizing operator, to
take this into account, and out-Goedel the new machine,
Goedelizing operator and all. This has, in fact, proved
to be the case. Even if we adjoin to a formal system the
infinite set of axioms consisting of the successive
Goedelian formulae, the resulting system is still
incomplete, and contains a formula which cannot be
proved-in-the-system, although a rational being can,
standing outside the system, see that it is true.^{2} We had expected this, for
even if an infinite set of axioms were added, they would
have to be specified by some finite rule or
specification, and this further rule or specification
could then be taken into account by a mind considering
the enlarged formal system. In a sense, just because the
mind has the last word, it can always pick a hole in any
formal system presented to it as a model of its own
workings. The [261] mechanical model must be, in some
sense, finite and definite: and then the mind can always
go one better.

This is the answer to one objection put forward by
Turing.^{3} He argues that the
limitation to the powers of a machine do not amount to
anything much. Although each individual machine is
incapable of getting the right answer to some questions,
after all each individual human being is fallible also:
and in any case "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." But this is not the point. We are not
discussing whether machines or minds are superior, but
whether they are the same. In some respect machines are
undoubtedly superior to human minds; and the question on
which they are stumped is admittedly, a rather niggling,
even (118) trivial, question. But it is enough, enough
to show that the machine is *not the same* as a
mind. True, the machine can do many things that a human
mind cannot do: but if there is of necessity something
that the machine cannot do, though the mind can, then,
however trivial the matter is, we cannot equate the two,
and cannot hope ever to have a mechanical model that will
adequately represent the mind. Nor does it signify that
it is only an individual machine we have triumphed over:
for the triumph is not over only *an* individual
machine, but over *any* individual that anybody
cares to specify---in Latin {50} *quivis* or
*quilibet*, not *quidam*---and a mechanical
model of a mind must be an individual machine. Although
it is true that any particular "triumph" of a mind over a
machine could be "trumped" by another machine able to
produce the answer the first machine could not produce,
so that "there is no question of triumphing
simultaneously over all machines", yet this is
irrelevant. What is at issue is not the unequal contest
between one mind and all machines, but whether there
could be any, single, machine that could do all a mind
can do. For the mechanist thesis to hold water, it must
be possible, in principle, to produce a model, a single
model, which can do everything the mind can do. It is
like a game.^{4} The mechanist
has first turn. He produces *a*---*any*, but
only a *definite one*---mechanical model of the
mind. I point to something that it cannot do, but the
mind can. The mechanist is free to modify his example,
but each time he does so, I am entitled to look for
defects in the revised model. If the mechanist can
devise a model that I cannot find fault with, his [262]
thesis is established: if he cannot, then it is not
proven: and since---as it turns out-he necessarily
cannot, it is refuted. To succeed, he must be able to
produce some definite mechanical model of the
mind---anyone he likes,
but one he can specify, and will stick to.
But since he cannot, in principle cannot, produce any
mechanical model that is adequate, even though the point
of failure is a minor one, he is bound to fail, and
mechanism must be false.

Deeper objections can still be made. Goedel's theorem
applies to deductive systems, and human beings are not
confined to making only deductive inferences. Goedel's
theorem applies only to consistent systems, and one may
have doubts about how far it is permissible to assume
that human beings are consistent. Goedel's theorem
applies only to formal systems, and there is no *a
priori* bound to human ingenuity which rules out the
possibility of our contriving some replica of humanity
which was not representable by a formal system.

Human beings are not confined to making deductive
inferences, and it has been urged by
C.G. Hempel^{5}
and Hartley Rogers^{6}
that a fair model of the
mind would have to allow for the possibility of making
non-deductive inferences, and these might provide a way
of escaping the Goedel result. Hartley Rogers makes the
specific suggestion that the {51} machine should be
programmed to entertain various propositions which had
not been proved or disproved, and on occasion to add them
to its list of axioms. Fermat's last theorem or
Goldbach's conjecture might thus be added. If
subsequently their inclusion was found to lead to a
contradiction, they would be dropped again, and indeed in
those circumstances their negations would be added to the
list of theorems. In this sort of way a machine might
well be constructed which was able to produce as true
certain formulae which could not be proved from its
axioms according to its rules of inference. And
therefore the method of demonstrating the mind's
superiority over the machine might no longer work.

The construction of such a machine, however, presents difficulties. It cannot accept all unprovable formulae, and add them to its axioms, or it will find itself accepting both the Goedelian formula and its negation, and so be inconsistent. Nor would it do if it accepted the first of each pair of undecidable formulae, and, having added that to its axioms, would no longer regard its negation as undecidable, and so would never accept it too: for it might happen on the wrong member of the pair: it might accept the negation of the Goedelian formula rather than the Goedelian formula itself. And the system constituted [263] by a normal set of axioms with the negation of the Goedelian formula adjoined, although not inconsistent, is an unsound system, not admitting of the natural interpretation. It is something like non- Desarguian geometries in two dimensions: not actually inconsistent, but rather wrong, sufficiently much so to disqualify it from serious consideration. A machine which was liable to infelicities of that kind would be no model for the human mind.

It becomes clear that rather careful criteria of
selection of unprovable formulae will be needed. Hartley
Rogers suggests some possible ones. But once we have
rules generating new axioms, even if the axioms generated
are only provisionally accepted, and are liable to be
dropped again if they are found to lead to inconsistency,
then we can set about doing a Goedel on this system, as on
any other. We are in the same case as when we had a rule
generating the infinite set of Goedelian formulae as
axioms. In short, however a machine is designed, it must
proceed either at random or according to definite rules.
In so far as its procedure is random, we cannot outsmart
it: (120) but its performance is not going to be a
convincing parody of intelligent behaviour: in so far as
its procedure is in accordance with definite rules, the
Goedel method can {52} be used to produce a formula which
the machine, according to those rules, cannot assert as
true, although we, standing outside the system, can see
it to be true.^{7}

Goedel's theorem applies only to consistent systems.
All that we can prove *formally* is that *if*
the system is consistent, then the Goedelian formula is
unprovable-in-the-system. To be able to say
categorically that the Goedelian formula is unprovable-in-
the-system, and therefore true, we must not only be
dealing with a consistent system, but be able to say that
it is consistent. And, as Goedel showed in his second
theorem---a corollary of his first---it is impossible to
prove in a consistent system that that system is
consistent. Thus in order to fault the machine by
producing a formula of which we can say both that it is
true and that the machine cannot produce it as true, we
have to be able to say that the machine (or, rather, its
corresponding formal system) is consistent; and there is
no absolute proof of this. All we can do is to examine
the machine and see if it appears consistent. There
always remains the possibility of some inconsistency not
yet detected. At best we can say that the machine is
consistent, provided we are. But by what right can we do
this? Goedel's second [264] theorem seems to show that a
man cannot assert his own consistency, and so Hartley
Rogers^{8} argues that we
cannot really use Goedel's first theorem to counter the
mechanist thesis unless we can say that "there are
distinctive attributes which enable a human being to
transcend this last limitation and assert his own
consistency while still remaining consistent".

A man's untutored reaction if his consistency is
questioned is to affirm it vehemently: but this, in view
of Goedel's second theorem, is taken by some philosophers
as evidence of his actual inconsistency. Professor
Putnam^{9} has suggested that
human beings are machines, but inconsistent machines. If
a machine were wired to correspond to an inconsistent
system, then there would be no well-formed formula which
it could not produce as true; and so in no way could it
be proved to be inferior to a human being. Nor could we
make its inconsistency a reproach to it---are not men
inconsistent too? Certainly women are, and politicians;
and {53} even male non-politicians (121) contradict
themselves sometimes, and a single inconsistency is
enough to make a system inconsistent.

The fact that we are all sometimes inconsistent cannot
be gainsaid, but from this it does not follow that we are
tantamount to inconsistent systems. Our inconsistencies
are mistakes rather than set policies. They correspond
to the occasional malfunctioning of a machine, not its
normal scheme of operations. Witness to this that we
eschew inconsistencies when we recognize them for what
they are. If we really were inconsistent machines, we
should remain content with our inconsistencies, and would
happily affirm both halves of a contradiction. Moreover,
we would be prepared to say absolutely anything---which
we are not. It is easily shown^{10} that in an inconsistent formal
system everything is provable, and the requirement of
consistency turns out to be just that not everything can
be proved in it---it is not the case that "anything
goes." This surely is a characteristic of the mental
operations of human beings: they are selective: they do
discriminate between favoured---true---and unfavoured---
false---statements: when a person is prepared to say
anything, and is prepared to contradict himself without
any qualm or repugnance, then he is adjudged to have
"lost his mind". Human beings, although not perfectly
consistent, are not so much inconsistent as fallible.

A fallible but self-correcting machine would still be
subject to Goedel's results. Only a fundamentally
inconsistent machine would [265] escape. Could we have a
fundamentally inconsistent, but at the same time self-
correcting machine, which both would be free of Goedel's
results and yet would not be trivial and entirely unlike
a human being?
A machine with a rather *recherché:*
inconsistency wired into it, so that for all normal
purposes it was consistent, but when presented with the
Goedelian sentence was able to prove it?

There are all sorts of ways in which undesirable
proofs might be obviated. We might have a rule that
whenever we have proved *p* and not-*p*, we
examine their proofs and reject the longer. Or we might
arrange the axioms and rules of inference in a certain
order, and when a proof leading to an inconsistency is
proffered, see what axioms and rules are required for it,
and reject that axiom or rule which comes last in the
ordering. In some such way as this we could have an
inconsistent system, with a stop-rule, so that the
inconsistency was never allowed to come out in the form
of an inconsistent formula.

The suggestion at first sight seems attractive: yet there is something deeply wrong. Even though we might preserve the facade of consistency {54} by having a rule that whenever two inconsistent formulae (122) appear we were to reject the one with the longer proof, yet such a rule would be repugnant in our logical sense. Even the less arbitrary suggestions are too arbitrary. No longer does the system operate with certain definite rules of inference on certain definite formulae. Instead, the rules apply, the axioms are true, provided . . . we do not happen to find it inconvenient. We no longer know where we stand. One application of the rule of Modus Ponens may be accepted while another is rejected: on one occasion an axiom may be true, or another apparently false. The system will have ceased to be a formal logical system, and the machine will barely qualify for the title of a model for the mind. For it will be far from resembling the mind in its operations: the mind does indeed try out dubious axioms and rules of inference; but if they are found to lead to contradiction, they are rejected altogether. We try out axioms and rules of inference provisionally---true: but we do not keep them, once they are found to lead to contradictions. We may seek to replace them with others, we may feel that our formalization is at fault, and that though some axiom or rule of inference of this sort is required, we have not been able to formulate it quite correctly: but we do not retain the, faulty formulations without modification, merely with the proviso that when the argument leads to a contradiction we refuse to follow it. To do this would be utterly irrational. We should be in the position that on some occasions when supplied with the premisses of a Modus Ponens, say, we applied the rule and allowed the conclusion, and [266] on other occasions we refused to apply the rule, and disallowed the conclusion. A person, or a machine, which did this without being able to give a good reason for so doing, would be accounted arbitrary and irrational. It is part of the concept of "arguments" or "reasons" that they are in some sense general and universal: that if Modus Ponens is a valid method of arguing when I am establishing a desired conclusion, it is a valid method also when you, my opponent, are establishing a conclusion I do not want to accept. We cannot pick and choose the times when a form of argument is to be valid; not if we are to be reasonable. It is of course true, that with our informal arguments, which are not fully formalized, we do distinguish between arguments which are at first sight similar, adding further reasons why they are nonetheless not really similar: and it might be maintained that a {55} machine might likewise be entitled to distinguish between arguments at first sight similar, if it had good reason for doing so. And it might further be maintained that the machine had good reason for rejecting those patterns of argument it did reject, indeed the best of reasons, namely the avoidance of contradiction. But that, if it is a reason at all, is too good a reason. We do not lay it to a man's credit that he avoids contradiction merely by refusing to accept those arguments which would lead him to it, for no other (123) reason than that otherwise he would be led to it. Special pleading rather than sound argument is the name for that type of reasoning. No credit accrues to a man who, clever enough to see a few moves of argument ahead, avoids being brought to acknowledge his own inconsistency, by stonewalling as soon as he sees where the argument will end. Rather, we account him inconsistent too, not, in his case, because he affirmed and denied the same proposition, but because he used and refused to use the same rule of inference. A stop-rule on actually enunciating an inconsistency is not enough to save an inconsistent machine from being called inconsistent.

The possibility yet remains that we are inconsistent,
and there is no stop-rule, but the inconsistency is so
*recherché:* that it has never turned up. After all,
naive set-theory, which was deeply embedded in common-
sense ways of thinking did turn out to be inconsistent.
Can we be sure that a similar fate is not in store for
simple arithmetic too? In a sense we cannot, in spite of
our great feeling of certitude that our system of whole
numbers which can be added and multiplied together is
never going to prove inconsistent. It is just
conceivable we might find we had formalized it
incorrectly. If we had, we should try and formulate anew
our intuitive concept of number, as we have our intuitive
concept of a set. If we did this, we should of course
recast our system: our present axioms and rules of
inference would [267] be utterly rejected: there would be
no question of our using and not using them in an
"inconsistent" fashion. We should, once we had recast
the system, be in the same position as we are now,
possessed of a system believed to be consistent, but not
provably so. But then could there not be some other
inconsistency? It is indeed a possibility. But again no
inconsistency once detected will be tolerated. We are
determined not to be inconsistent, and are resolved to
root out inconsistency, should any appear. Thus,
although we can never be completely certain or completely
free of the risk of having to think out our mathematics
again, the ultimate position must be one of two: either
we have a system of simple arithmetic which to the best
of our knowledge and belief is consistent: or there is no
such system possible. In the former case we are in the
same position as at present: in the {56} latter, if we
find that no system containing simple arithmetic can be
free of contradictions, we shall have to abandon not
merely the whole of mathematics and the mathematical
sciences, but the whole of thought.

It may still be maintained that although a man must in
this sense assume, he cannot properly affirm, his own
consistency without thereby belying his words. We may be
consistent; indeed we have every reason to hope that we
are: but a necessary modesty forbids us from saying so.
Yet this is not quite what Goedel's second theorem states.
Goedel has shown that in a consistent system a formula
(124) stating the consistency of the system cannot be
proved *in that system*. It follows that a machine,
if consistent, cannot produce as true an assertion of its
own consistency: hence also that a mind, *if it were
really a machine*, could not reach the conclusion that
it was a consistent one. For a mind which is not a
machine no such conclusion follows. All that Goedel has
proved is that a mind cannot produce a formal proof of
the consistency of a formal system inside the system
itself: but there is no objection to going outside the
system and no objection to producing informal arguments
for the consistency either of a formal system or of
something less formal and less systematized. Such
informal arguments will not be able to be completely
formalized: but then the whole tenor of Goedel's results
is that we ought not to ask, and cannot obtain, complete
formalization. And although it would have been nice if
we could have obtained them, since completely formalized
arguments are more coercive than informal ones, yet since
we cannot have all our arguments cast into that form, we
must not hold it against informal arguments that they are
informal or regard them all as utterly worthless. It
therefore seems to me both proper and reasonable for a
mind to assert its own consistency: proper, because
although machines, as we might have expected, are [268]
unable to reflect fully on their own performance and
powers, yet to be able to be self-conscious in this way
is just what we expect of minds: and reasonable, for the
reasons given. Not only can we fairly say simply that we
*know* we are consistent, apart from our mistakes,
but we must in any case *assume* that we are, if
thought is to be possible at all; moreover we are
selective, we will not, as inconsistent machines would,
say anything and everything whatsoever: and finally we
can, in a sense, *decide* to be consistent, in the
sense that we can resolve not to tolerate inconsistencies
in our thinking and speaking, and to eliminate them, if
ever they should appear, by withdrawing and cancelling
one limb of the contradiction.

We can see how we might almost have expected Goedel's theorem to distinguish self-conscious beings from inanimate objects. The essence of {57} the Goedelian formula is that it is self-referring. It says that "This formula is unprovable-in-this-system". When carried over to a machine, the formula is specified in terms which depend on the particular machine in question. The machine is being asked a question about its own processes. We are asking it to be self-conscious, and say what things it can and cannot do. Such questions notoriously lead to paradox. At one's first and simplest attempts to philosophize, one becomes entangled in questions of whether when one knows something one knows that one knows it, and what, when one is thinking of oneself, is being thought about, and what is doing the thinking. After one has been puzzled and bruised by this (125) problem for a long time, one learns not to press these questions: the concept of a conscious being is, implicitly, realized to be different from that of an unconscious object. In saying that a conscious being knows something, we are saying not only that he knows it, but that he knows that he knows it, and that he knows that he knows that he knows it, and so on, as long as we care to pose the question: there is, we recognize, an infinity here, but it is not an infinite regress in the bad sense, for it is the questions that peter out, as being pointless, rather than the answers. The questions are felt to be pointless because the concept contains within itself the idea of being able to go on answering such questions indefinitely. Although conscious beings have the power of going on, we do not wish to exhibit this simply as a succession of tasks they are able to perform, nor do we see the mind as an infinite sequence of selves and super-selves and super-superselves. Rather, we insist that a conscious being is a unity, and though we talk about parts of the mind, we do so only as a metaphor, and will not allow it to be taken literally.

The paradoxes of consciousness arise because a conscious being can be aware of itself, as well as of other things, and yet cannot [269] really be construed as being divisible into parts. It means that a conscious being can deal with Goedelian questions in a way in which a machine cannot, because a conscious being can both consider itself and its performance and yet not be other than that which did the performance. A machine can be made in a manner of speaking to "consider" its own performance, but it cannot take this "into account" without thereby becoming a different machine, namely the old machine with a "new part" added. But it is inherent in our idea of a conscious mind that it can reflect upon itself and criticize its own performances, and no extra part is required to do this: it is already complete, and has no Achilles' heel.

The thesis thus begins to become more a matter of
conceptual analysis {58}than mathematical discovery.
This is borne out by considering another argument put
forward by Turing.^{11} So
far, we have constructed only fairly simple and
predictable artefacts. When we increase the complexity
of our machines there may, perhaps, be surprises in store
for us. He draws a parallel with a fission pile. Below
a certain "critical" size, nothing much happens: but
above the critical size, the sparks begin to fly. So
too, perhaps, with brains and machines. Most brains and
all machines are, at present, "subcritical"---they react
to incoming stimuli in a stodgy and uninteresting way,
have no ideas of their own, can produce only stock
responses ---but a few brains at present, and possibly
some machines in the future, are super-critical, and
scintillate on their own account. (126) Turing is
suggesting that it is only a matter of complexity, and
that above a certain level of complexity a qualitative
difference appears, so that 44 super-critical" machines
will be quite unlike the simple ones hitherto envisaged.

This may be so. Complexity often does introduce
qualitative differences. Although it sounds implausible,
it might turn out that above a certain level of
complexity, a machine ceased to be predictable, even in
principle, and started doing things on its own account,
or, to use a very revealing phrase, it might begin to
have a mind of its own. It might begin to have a mind of
its own. It would begin to have a mind of its own when
it was no longer entirely predictable and entirely
docile, but was capable of doing things which we
recognized as intelligent, and not just mistakes or
random shots, but which we had not programmed into it.
But then it would cease to be a machine, within the
meaning of the act. What is at stake in the mechanist
debate is not how minds are, or might be, brought into
being, but how they operate. It is essential for the
mechanist thesis that the mechanical model of the mind
shall operate according [270] to "mechanical principles",
that is, that we can understand the operation of the
whole in terms of the operations of its parts, and the
operation of each part either shall be determined by its
initial state and the construction of the machine, or
shall be a random choice between a determinate number of
determinate operations. If the mechanist produces a
machine which is so complicated that this ceases to hold
good of it, then it is no longer a machine for the
purposes of our discussion, no matter how it was
constructed. We should say, rather, that he had created
a mind, in the same sort of sense as we procreate people
at present. There would then be two ways of bringing new
minds into the world, the traditional way, by begetting
children born of women, and a new way by constructing
very, very complicated systems of, say, valves {59} and
relays. When talking of the second way, we should take
care to stress that although what was created looked like
a machine, it was not one really, because it was not just
the total of its parts. One could not tell what it was
going to do merely by knowing the way in which it was
built up and the initial state of its parts: one could
not even tell the limits of what it could do, for even
when presented with a Goedel-type question, it got the
answer right. In fact we should say briefly that any
system which was not floored by the Goedel question was
*eo ipso* not a Turing machine, i.e., not a machine
within the meaning of the act.

If the proof of the falsity of mechanism is valid, it
is of the greatest consequence for the whole of
philosophy. Since the time of Newton, the bogey of
mechanist determinism has obsessed philosophers. If we
were to be scientific, it seemed that we must look on
human beings as (127) determined automata, and not as
autonomous moral agents; if we were to be moral, it
seemed that we must deny science its due, set an
arbitrary limit to its progress in understanding human
neurophysiology, and take refuge in obscurantist
mysticism. Not even Kant could resolve the tension
between the two standpoints. But now, though many
arguments against human freedom still remain, the
argument from mechanism, perhaps the most compelling
argument of them all, has lost its power. No longer on
this count will it be incumbent on the natural
philosopher to deny freedom in the name of science: no
longer will the moralist feel the urge to abolish
knowledge to make room for faith. We can even begin to
see how there could be room for morality, without its
being necessary to abolish or even to circumscribe the
province of science. Our argument has set no limits to
scientific enquiry: it will still be possible to
investigate the working of the brain. It will still be
possible to produce mechanical models of the mind. Only,
now we can see that no mechanical model will be
completely adequate, nor any explanations [271] in purely
mechanist terms. We can produce models and explanations,
and they will be illuminating: but, however far they go,
there will always remain more to be said. There is no
arbitrary bound to scientific enquiry: but no scientific
enquiry can ever exhaust the infinite variety of the
human mind.^{12}

1. See A. M. Turing, "Computing Machinery and Intelligence,"