This is a long quote from near the beginning of What Does It All Mean?, a 1987 book by
the American philosopher Thomas Nagel.
In this passage, Nagel articulates a philosophical problem that is standard for introductory
philosophy courses.
If you have read Descartes, you will recognize that Nagel’s approach here shares a key
Cartesian assumption: that the problem of skepticism must be resolved before we can
make any knowledge claims about anything outside our own minds.
Put another way, philosophers must solve the epistemological problem of justifying knowledge,
before they can entertain theories about metaphysics, ethics, or human nature.
As you listen and read, pay attention to the logic of Nagel's argument, and to the assumptions
about human experience it requires.
Here’s the quote.
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
United States, or that water is H2O.
Everything else is farther away from you than your inner experiences and thoughts, and reaches
you only through them.
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 experiences 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 Kansas City.
We also assume that normal dreams depend on what is happening in the dreamer's brain while
he sleeps.
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.