How do we know anything?
Thomas
Nagel
If you think about it, the inside of your own mind is the only
thing you can be sure of.
Whatever you believe - whether it's about the sun, moon,
and stars, the house and neighborhood in which you
live, history, science, other people, even the existence of your own body - is
based on your experiences and thoughts, feelings and sense impressions. That's all you have to go on directly,
whether you see the book in your hands, or feel the floor under your feet, or
remember that George Washington was the first president of the
Ordinarily you have no doubts about the existence of the floor
under your feet, or the tree outside the window, or your own teeth. In fact most of the time
you don't even think about the mental states that make you aware of those
things: you seem to be aware of them directly.
But how do you know they really exist?
If you try to argue that
there must be an external physical world, because you wouldn’t see
buildings, people or stars unless there were things out there that reflected or
shed light into your eyes and caused your visual experiences, the reply is
obvious: How do you know that? It’s just another claim about the external
world and your relation to it, and has to be based on the evidence of your
senses. But you can rely on that specific evidence
about how visual expereicnes are caused only if you
can already rely in general on the contents of your mind to tell you about the
external world. And that is exactly what
has been called into question. If
you try to prove the reliability of your impressions by appealing to your
impressions, you're arguing in a circle and won't get anywhere.
Would things seem any different to you if in fact all these things
existed only in your mind - if everything you took to be the real world outside
was just a giant dream or hallucination, from which you will never wake
up? If it were like that, then of course
you couldn't wake up, as you can from a dream, because it would mean there was
no "real: world to wake up into. So
it wouldn't be exactly like a normal dream or hallucination. As we usually think of dreams, they go on in
the minds of people who are actually lying in a real bed in a real house, even
if in the dream they are running away from a homicidal lawnmower through the
streets of
But couldn't all your experiences be like a giant dream with no external world outside it? How can you know that isn't what's going
on? If all your experience were a dream
with nothing outside, then any evidence you tried to use to prove to yourself
that there was an outside world would just be part of the dream. If you knocked on the table or pinched
yourself, you would hear the knock and feel the pinch, but that would be just
one more thing going on inside your mind like everything else. It's no use: If you want to find out whether
what's inside your mind is any guide to what's outside your mind, you can't
depend on how things seem - from inside your mind - to give you the answer.
But what else is there to depend on? All your evidence about anything has to come
through your mind - whether in the form of perception, the testimony of books
and other people, or memory - and it is entirely consistent with everything
you're aware of that nothing at all exists except the inside of your mind. It's even possible that you don't have a body
or a brain - since your beliefs about that come only through the evidence of your
senses. You've never seen your brain -
you just assume that everybody has one - but even if you had seen it, or
thought you had, that would have been just another visual experience. Maybe you, the subject of experience, are the
only thing that exists, and there is no physical world at all - no stars, no earth, no human
bodies. Maybe there isn't even any
space.
The most radical conclusion to draw from this would be that your
mind is the only thing that exists. This
view is called solipsism. It is a very
lonely view, and not too many people have held it. As you can tell from that remark, I don't
hold it myself. If I were a solipsist I
probably wouldn't be writing this book, since I wouldn't believe there was
anybody else to read it. On the other
hand, perhaps I would write it to make my inner life more interesting, by
including the impression of the appearance of the book in print, of other
people reading it and telling me their reactions, and so forth. I might even get the impression of royalties,
if I'm lucky.
Perhaps you are a solipsist: in that case you will regard this
book as a product of your own mind, coming into existence in your experience as
you read it. Obviously nothing I can say
can prove to you that I really exist, or that the book as a physical object
exists.
On the other hand, to conclude that you are the only thing that
exists is more than the evidence warrants.
You can't know on the basis of what's in your mind that there's no world
outside it. Perhaps the right conclusion
is the more modest one that you don't know anything beyond your impressions and
experiences. There may or may not be an
external world, and if there is it may or may not be completely different from
how it seems to you - there's no way for you to tell. This view is called skepticism
about the external world.
An even stronger form of skepticism is
possible. Similar arguments seem to show
that you don't know anything even about your own past existence and
experiences, since all you have to go on are the present contents of your mind,
including memory impressions. If you
can't be sure that the world outside your mind exists now, how can you be sure
that you yourself existed before now?
How do you know you didn't just come into existence a few minutes ago,
complete with all your present memories?
The only evidence that you couldn't have come into existence a few
minutes ago depends on beliefs about how people and their memories are
produced, which rely in turn on beliefs about what has happened in the
past. But to rely on those beliefs to
prove that you
existed in the past would again be to argue in a circle. You would be assuming
the reality of the past to prove the reality of the past.
It seems that you are stuck with nothing you can be sure of except
the contents of your own mind at the present moment. And it seems that anything you try to do to
argue your way out of this predicament will fail, because the argument will
have to assume what you are trying to prove - the existence of the external world
beyond your mind.
Suppose, for instance, you argue that there must be an external
world, because it is incredible that you should be having all these experiences
without there being some explanation in terms of external causes. The skeptic can
make two replies. First, even if there
are external causes, how can you tell from the contents of your experience what
those causes are like? You've never
observed any of them directly. Second,
what is the basis of your idea that everything has to have an explanation? It's true that in your normal, nonphilosophical conception of the world, processes like
those which go on in your mind are caused, at least in part, by other things
outside them. But you can't assume that
this is true if what you're trying to figure out is how you know anything about
the world outside your mind. And there
is no way to prove such a principle just by looking at what's inside your
mind. However plausible the principle
may seem to you, what reason do you have to believe that it applies to the
world?
Science won't help us with this problem either, though it might
seem to. In ordinary scientific
thinking, we rely on general principles of explanation to pass from the way the
world first seems to us to a different conception of what it is really like. We try to explain the appearances in terms of
a theory that describes the reality behind them, a reality that we can't
observe directly. That is how physics
and chemistry conclude that all the things we see around us are composed of
invisibly small atoms. Could we argue
that the general belief in the external world has the same kind of scientific
backing as the belief in atoms?
The skeptic's answer is that the process
of scientific reasoning raises the same skeptical problem we have been
considering all along: Science is just as vulnerable as perception. How can we know that the world outside our
minds corresponds to our ideas of what would be a good theoretical explanation
of our observations? If we can't
establish the reliability of our sense experiences in relation to the external
world, there's no reason to think we can rely on our scientific theories
either.
There is another very different response to the problem. Some would argue that radical skepticism of the kind I have been talking about is
meaningless, because the idea of an external reality that no one could ever
discover is meaningless. The argument is
that a dream, for instance, has to be something from which you can wake up to
discover that you have been asleep; a hallucination has to be something which
others (or you later) can see is not really there. Impressions and appearances that do not
correspond to reality must be contrasted with others that do correspond to
reality, or else the contrast between appearance and reality is meaningless.
According to this view, the idea of a dream from which you can
never wake up is not the idea of a dream at all: it is the idea of reality -
the real world in which you live. Our
idea of the things that exist is just our idea of what we can observe. (This view is sometimes called verificationism.)
Sometimes our observations are mistaken, but that means they can be
corrected by other observations -as when you wake up from a dream or discover
that what you thought was a snake was just a shadow on the grass. But without some possibility of a correct
view of how things are (either yours or someone else's), the thought that your
impressions of the world are not true is meaningless.
If this is right, then the skeptic is
kidding himself if he thinks he can imagine that the only thing that exists is
his own mind. He is kidding himself,
because it couldn't be true that the physical world doesn't really exist,
unless somebody could observe that it doesn't exist. And what the skeptic
is trying to imagine is precisely that there is no one to observe that or
anything else - except of course the skeptic himself,
and all he can observe is the inside of his own mind. So solipsism is meaningless. It tries to
subtract the external world from the totality of my impressions; but it fails,
because if the external world is subtracted, they stop being mere impressions,
and become instead perceptions of reality.
Is this argument against solipsism and skepticism
any good? Not unless reality can be
defined as what we can observe. But are
we really unable to understand the idea of a real world, or a fact about
reality, that can't be observed by anyone, human or otherwise?
The skeptic will claim that if there is
an external world, the things in it are observable because they exist, and not
the other way around: that existence isn't the same thing as observability. And
although we get the idea of dreams and hallucinations from cases where we think
we can observe the contrast between our experiences and reality, it certainly
seems as if the same idea can be extended to cases where the reality is not
observable.
If that is right, it seems to follow that it is not meaningless to
think that the world might consist of nothing but the inside of your mind,
though neither you nor anyone else could find out that this was true. And if this is not meaningless, but is a
possibility you must consider, there seems no way to prove that it is false,
without arguing in a circle. So there
may be no way out of the cage of your own mind.
This is sometimes called the egocentric predicament.
And yet, after all this has been said, I have to admit it is
practically impossible to believe seriously that all the things in the world
around you might not really exist. Our
acceptance of the external world is instinctive and powerful
: we cannot just get rid of it by philosophical arguments. Not only do we go on acting as if other
people and things exist: we believe that
they do, even after we've gone through the arguments which appear to show we
have no grounds for this belief (We may have grounds, within the overall system
of our beliefs about the world, for more particular beliefs about the existence
of particular things: like a mouse in the breadbox, for example. But that is different. It assumes the existence of the external
world.)
If a belief in the world outside our minds comes so naturally to
us, perhaps we don't need grounds for it.
We can just let it be and hope that we're right. And that in fact is what most people do after
giving up the attempt to prove it : even if they can't
give reasons against skepticism, they can't live with
it either. But this means that we hold
on to most of our ordinary beliefs about the world in face of the fact that (a)
they might be completely false, and (b) we have no basis for ruling out that
possibility.
We are left then with three questions:
• Is it a meaningful possibility that the inside of your mind is
the only thing that exists - or that even if there is a world outside your
mind, it is totally unlike what you believe it to be?
• If these things are possible, do you have any way of proving
to yourself that they are not actually true?
• If you can't prove that anything exists outside your own mind,
is it all right to go on believing in the external world anyway?