REASON, TRUTH AND HISTORY
Hilary Putnam
CAMBRIDGE
UNIVERSITY PRESS
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© Cambridge University Press 1981
This book is in copyright. Subject to statutory exception
and to the provisions of relevant collective licensing agreements,
no reproduction of any part may take place without
the written permission of Cambridge University Press.
First published 1981
Reprinted 1982, 1985, 1986, 1987, 1989, 1990, 1991, 1992, 1993,
1994, 1995, 1997, 1998
Printed in the United States of America
Typeset in Sabon
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1
Brains in a vat
An ant is crawling on a patch of sand. As it crawls, it traces a
line in the sand. By pure chance the line that it traces curves and
recrosses itself in such a way that it ends up looking like a rec-
ognizable caricature of Winston Churchill. Has the ant traced a
picture of Winston Churchill, a picture that depicts Churchill?
Most people would say, on a little reflection, that it has not.
The ant, after all, has never seen Churchill, or even a picture of
Churchill, and it had no intention of depicting Churchill. It sim-
ply traced a line (and even that was unintentional), a line that
we can 'see as' a picture of Churchill.
We can express this by saying that the line is not 'in itself a
representation1 of anything rather than anything else. Similarity
(of a certain very complicated sort) to the features of Winston
Churchill is not sufficient to make something represent or refer
to Churchill. Nor is it necessary: in our community the printed
shape 'Winston Churchill', the spoken words 'Winston Chur-
chill', and many other things are used to represent Churchill
(though not pictorially), while not having the sort of similarity
1 In this book the terms 'representation' and 'reference' always refer to a
relation between a word (or other sort of sign, symbol, or
representation) and something that actually exists (i.e. not just an 'object
of thought'). There is a sense of 'refer' in which I can 'refer' to what does
not exist; this is not the sense in which 'refer' is used here. An older
word for what I call 'representation' or 'reference' is denotation.
Secondly, I follow the custom of modern logicians and use 'exist' to
mean 'exist in the past, present, or future'. Thus Winston Churchill
'exists', and we can 'refer to' or 'represent' Winston Churchill, even
though he is no longer alive.
2 Brains in a vat
to Churchill that a picture — even a line drawing — has. If simi-
larity is not necessary or sufficient to make something represent
something else, how can anything be necessary or sufficient for
this purpose? How on earth can one thing represent (or 'stand
for', etc.) a different thing?
The answer may seem easy. Suppose the ant had seen Winston
Churchill, and suppose that it had the intelligence and skill to
draw a picture of him. Suppose it produced the caricature inten-
tionally. Then the line would have represented Churchill.
On the other hand, suppose the line had the shape WINSTON
CHURCHILL. And suppose this was just accident (ignoring the
improbability involved). Then the 'printed shape' WINSTON
CHURCHILL would not have represented Churchill, although
that printed shape does represent Churchill when it occurs in
almost any book today.
So it may seem that what is necessary for representation, or
what is mainly necessary for representation, is intention.
But to have the intention that anything, even private lan-
guage (even the words 'Winston Churchill' spoken in my mind
and not out loud), should represent Churchill, I must have been
able to think about Churchill in the first place. If lines in the
sand, noises, etc., cannot 'in themselves' represent anything, then
how is it that thought forms can 'in themselves' represent any-
thing? Or can they? How can thought reach out and 'grasp'
what is external?
Some philosophers have, in the past, leaped from this sort of
consideration to what they take to be a proof that the mind is
essentially non-physical in nature. The argument is simple; what
we said about the ant's curve applies to any physical object. No
physical object can, in itself, refer to one thing rather than to
another; nevertheless, thoughts in the mind obviously do suc-
ceed in referring to one thing rather than another. So thoughts
(and hence the mind) are of an essentially different nature than
physical objects. Thoughts have the characteristic of intention-
ality — they can refer to something else; nothing physical has
'intentionality', save as that intentionality is derivative from
some employment of that physical thing by a mind. Or so it is
claimed. This is too quick; just postulating mysterious powers of
mind solves nothing. But the problem is very real. How is inten-
tionahty, reference, possible?
Brains in a vat 3
Magical theories of reference
We saw that the ant's 'picture' has no necessary connection with
Winston Churchill. The mere fact that the 'picture' bears a
'resemblance' to Churchill does not make it into a real picture,
nor does it make it a representation of Churchill. Unless the ant
is an intelligent ant (which it isn't) and knows about Churchill
(which it doesn't), the curve it traced is not a picture or even a
representation of anything. Some primitive people believe that
some representations (in particular, names) have a necessary
connection with their bearers; that to know the 'true name' of
someone or something gives one power over it. This power
comes from the magical connection between the name and the
bearer of the name; once one realizes that a name only has a
contextual, contingent, conventional connection with its bearer,
it is hard to see why knowledge of the name should have any
mystical significance.
What is important to realize is that what goes for physical
pictures also goes for mental images, and for mental representa-
tions in general; mental representations no more have a neces-
sary connection with what they represent than physical represen-
tations do. The contrary supposition is a survival of magical
thinking.
Perhaps the point is easiest to grasp in the case of mental
images. (Perhaps the first philosopher to grasp the enormous sig-
nificance of this point, even if he was not the first to actually
make it, was Wittgenstein.) Suppose there is a planet somewhere
on which human beings have evolved (or been deposited by alien
spacemen, or what have you). Suppose these humans, although
otherwise like us, have never seen trees. Suppose they have never
imagined trees (perhaps vegetable life exists on their planet only
in the form of molds). Suppose one day a picture of a tree is
accidentally dropped on their planet by a spaceship which passes
on without having other contact with them. Imagine them puz-
zling over the picture. What in the world is this? All sorts of
speculations occur to them: a building, a canopy, even an animal
of some kind. But suppose they never come close to the truth.
For us the picture is a representation of a tree. For these
humans the picture only represents a strange object, nature and
function unknown. Suppose one of them has a mental image
4 Brains in a vat
which is exactly like one of my mental images of a tree as a result
of having seen the picture. His mental image is not a represen-
tation of a tree. It is only a representation of the strange object
(whatever it is) that the mysterious picture represents.
Still, someone might argue that the mental image is in fact a
representation of a tree, if only because the picture which caused
this mental image was itself a representation of a tree to begin
with. There is a causal chain from actual trees to the mental
image even if it is a very strange one.
But even this causal chain can be imagined absent. Suppose
the 'picture of the tree' that the spaceship dropped was not really
a picture of a tree, but the accidental result of some spilled
paints. Even if it looked exactly like a picture of a tree, it was, in
truth, no more a picture of a tree than the ant's 'caricature' of
Churchill was a picture of Churchill. We can even imagine that
the spaceship which dropped the 'picture' came from a planet
which knew nothing of trees. Then the humans would still have
mental images qualitatively identical with my image of a tree,
but they would not be images which represented a tree any more
than anything else.
The same thing is true of words. A discourse on paper might
seem to be a perfect description of trees, but if it was produced
by monkeys randomly hitting keys on a typewriter for millions
of years, then the words do not refer to anything. If there were
a person who memorized those words and said them in his mind
without understanding them, then they would not refer to any-
thing when thought in the mind, either.
Imagine the person who is saying those words in his mind has
been hypnotized. Suppose the words are in Japanese, and the per-
son has been told that he understands Japanese. Suppose that as
he thinks those words he has a 'feeling of understanding'.
(Although if someone broke into his train of thought and asked
him what the words he was thinking meant, he would discover
he couldn't say.) Perhaps the illusion would be so perfect that
the person could even fool a Japanese telepath! But if he
couldn't use the words in the right contexts, answer questions
about what he 'thought', etc., then he didn't understand them.
By combining these science fiction stories I have been telling,
we can contrive a case in which someone thinks words which are
in fact a description of trees in some language and simultane-
Brains in a vat S
ously has appropriate mental images, but neither understands
the words nor knows what a tree is. We can even imagine that
the mental images were caused by paint-spills (although the per-
son has been hypnotized to think that they are images of some-
thing appropriate to his thought — only, if he were asked, he
wouldn't be able to say of what). And we can imagine that the
language the person is thinking in is one neither the hypnotist
nor the person hypnotized has ever heard of — perhaps it is just
coincidence that these 'nonsense sentences', as the hypnotist sup-
poses them to be, are a description of trees in Japanese. In short,
everything passing before the person's mind might be qualita-
tively identical with what was passing through the mind of a
Japanese speaker who was really thinking about trees - but
none of it would refer to trees.
All of this is really impossible, of course, in the way that it is
really impossible that monkeys should by chance type out a copy
of Hamlet. That is to say that the probabilities against it are so
high as to mean it will never really happen (we think). But is is
not logically impossible, or even physically impossible. It could
happen (compatibly with physical law and, perhaps, compatibly
with actual conditions in the universe, if there are lots of intelli-
gent beings on other planets). And if it did happen, it would be
a striking demonstration of an important conceptual truth; that
even a large and complex system of representations, both verbal
and visual, still does not have an intrinsic, built-in, magical con-
nection with what it represents — a connection independent of
how it was caused and what the dispositions of the speaker or
thinker are. And this is true whether the system of representa-
tions (words and images, in the case of the example) is physically
realized - the words are written or spoken, and the pictures are
physical pictures - or only realized in the mind. Thought words
and mental pictures do not intrinsically represent what they are
about.
The case of the brains in a vat
Here is a science fiction possibility discussed by philosophers:
imagine that a human being (you can imagine this to be yourself)
has been subjected to an operation by an evil scientist. The per-
son's brain (your brain) has been removed from the body and
6 Brains in a vat
placed in a vat of nutrients which keeps the brain alive. The
nerve endings have been connected to a super-scientific com-
puter which causes the person whose brain it is to have the illu-
sion that everything is perfectly normal. There seem to be peo-
ple, objects, the sky, etc; but really all the person (you) is
experiencing is the result of electronic impulses travelling from
the computer to the nerve endings. ITie computer is so clever
that if the person tries to raise his hand, the feedback from the
computer will cause him to 'see' and 'feel' the hand being raised.
Moreover, by varying the program, the evil scientist can cause
the victim to 'experience' (or hallucinate) any situation or envi-
ronment the evil scientist wishes. He can also obliterate the
memory of the brain operation, so that the victim will seem to
himself to have always been in this environment. It can even
seem to the victim that he is sitting and reading these very words
about the amusing but quite absurd supposition that there is an
evil scientist who removes people's brains from their bodies and
places them in a vat of nutrients which keep the brains alive. The
nerve endings are supposed to be connected to a super-scientific
computer which causes the person whose brain it is to have the
illusion that . . .
When this sort of possibility is mentioned in a lecture on the
Theory of Knowledge, the purpose, of course, is to raise the clas-
sical problem of scepticism with respect to the external world in
a modem way. ( H o w do you know you aren't in this predica-
ment?) But this predicament is also a useful device for raising
issues about the mind/world relationship.
Instead of having just one brain in a vat, we could imagine
that all human beings (perhaps all sentient beings) are brains in
a vat (or nervous systems in a vat in case some beings with just
a minimal nervous system already count as 'sentient'). Of course,
the evil scientist would have to be outside - or would he? Per-
haps there is no evil scientist, perhaps (though this is absurd) the
universe just happens to consist of automatic machinery tending
a vat full of brains and nervous systems.
This time let us suppose that the automatic machinery is pro-
grammed to give us all a collective hallucination, rather than a
number of separate unrelated hallucinations. Thus, when I seem
to myself to be talking to you, you seem to yourself to be hearing
my words. Of course, it is not the case that my words actually
Brains in a vat 7
reach your ears - for you don't have (real) ears, nor do I have a
real mouth and tongue. Rather, when I produce my words, what
happens is that the efferent impulses travel from my brain to the
computer, which both causes me to 'hear' my own voice uttering
those words and 'feel' my tongue moving, etc., and causes you
to 'hear' my words, 'see' me speaking, etc. In this case, we are,
in a sense, actually in communication. I am not mistaken about
your real existence (only about the existence of your body and
the 'external world', apart from brains). From a certain point of
view, it doesn't even matter that 'the whole world' is a collective
hallucination; for you do, after all, really hear my words when
I speak to you, even if the mechanism isn't what we suppose it
to be. (Of course, if we were two lovers making love, rather than
just two people carrying on a conversation, then the suggestion
that it was just two brains in a vat might be disturbing.)
I want now to ask a question which will seem very silly and
obvious (at least to some people, including some very sophisti-
cated philosophers), but which will take us to real philosophical
depths rather quickly. Suppose this whole story were actually
true. Could we, if we were brains in a vat in this way, say or
think that we were?
I am going to argue that the answer is 'No, we couldn't.' In
fact, I am going to argue that the supposition that we are
actually brains in a vat, although it violates no physical law, and
is perfectly consistent with everything we have experienced, can-
not possibly be true. It cannot possibly be true, because it is, in
a certain way, self-refuting.
The argument I am going to present is an unusual one, and it
took me several years to convince myself that it is really right.
But it is a correct argument. What makes it seem so strange is
that it is connected with some of the very deepest issues in phi-
losophy. (It first occurred to me when I was thinking about a
theorem in modern logic, the 'Skolem-Lowenheim Theorem',
and I suddenly saw a connection between this theorem and some
arguments in Wittgenstein's Philosophical Investigations.)
A 'self-refuting supposition' is one whose truth implies its own
falsity. For example, consider the thesis that all general state-
ments are false. This is a general statement. So if it is true, then
it must be false. Hence, it is false. Sometimes a thesis is called
'self-refuting' if it is the supposition that the thesis is entertained
8 Brains in a vat
or enunciated that implies its falsity. For example, 'I do not exist'
is self-refuting if thought by me (for any 'me'). So one can be
certain that one oneself exists, if one thinks about it (as Des-
cartes argued).
What I shall show is that the supposition that we are brains in
a vat has just this property. If we can consider whether it is true
or false, then it is not true (I shall show). Hence it is not true.
Before I give the argument, let us consider why it seems so
strange that such an argument can be given (at least to philoso-
phers who subscribe to a 'copy' conception of truth). We con-
ceded that it is compatible with physical law that there should
be a world in which all sentient beings are brains in a vat. As
philosophers say, there is a 'possible world' in which all sentient
beings are brains in a vat. (This 'possible world' talk makes it
sound as if there is a place where any absurd supposition is true,
which is why it can be very misleading in philosophy.) The
humans in that possible world have exactly the same experiences
that we do. They think the same thoughts we do (at least, the
same words, images, thought-forms, etc., go through their
minds). Yet, I am claiming that there is an argument we can give
that shows we are not brains in a vat. How can there be? And
why couldn't the people in the possible world who really are
brains in a vat give it too?
The answer is going to be (basically) this: although the people
in that possible world can think and 'say' any words we can
think and say, they cannot (I claim) refer to what we can refer
to. In particular, they cannot think or say that they are brains in
a vat (even by thinking 'we are brains in a vat').
Turing's test
Suppose someone succeeds in inventing a computer which can
actually carry on an intelligent conversation with one (on as
many subjects as an intelligent person might). How can one
decide if the computer is 'conscious'?
The British logician Alan Turing proposed the following test:2
let someone carry on a conversation with the computer and a
conversation with a person whom he does not know. If he can-
2 A. M. Turing, 'Computing Machinery and Intelligence', Mind (1950),
reprinted in A. R. Anderson (ed.). Minds and Machines.
Brains in a vat 9
not tell which is the computer and which is the human being,
then (assume the test to be repeated a sufficient number of times
with different interlocutors) the computer is conscious. In short,
a computing
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