John asked:

About a month ago, I suddenly started thinking about how completely impossible it is to ever know
whether you are the only one who really exists, because one can't go into another's mind to see if
they are a conscious being like you are. What if everyone else is just in your imagination or
something like that? You can never know! I know this question seems strange, but the problem has
been haunting me ever since I thought of it and I would really like an answer. It has really been
consuming most of my thoughts during school (I'm 14) especially and is very distracting.

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Philosophers call this problem 'solipsism' from the Latin solus ipsethe sole self. There's two main
kinds of solipsism. The first kind is easier to defeat then the second kind.

The first kind is sometimes known as 'scepticism about other minds'. It is a selective kind of
scepticism, in that you don't question whether you are in a world of physical objects in space. That's a
given. The problem is that amongst these physical objects are living human bodies that make
apparently meaningful movements and utter apparently meaningful sounds. The question is, How do I
know that there is anything insidethe people I meet, how do I know that I am not the only real person
surrounded by perfectly disguised zombies?

I think I have an argument which can be used to defeat that question. It's not quite a knock-out punch,
but close. First, we have to set the scene. The person asking this question is a dualistwho believes
that there are basically two kinds of 'stuff' in the universe, physical stuff and mental stuff. The physical
stuff is all around me, but the only mental stuff I have direct knowledge of is my own mind, my own
consciousness. I can't be certain whether other human bodies have mental stuff inside them or not,
because the mind is something that can only be directly seenfrom the inside.

Descartes was a dualist. But he believed that my mental stuff, or rather my soulinteracts with my
physical body. A human body needs a soul to make it go. Without something 'in control' there would
no speech or intelligible action. (He thought that animals didn't have souls.) Now, if that's true, then it
provides a simple test for whether another person has a mind.

Today, dualists no longer believe in a soul that interacts with the brain. They hold that processes in
the brain are sufficient to account for the noises people utter and the physical movements they make.
But those processes alsogive rise to something inside, as a kind of by-product. Mental stuff is
produced by a functioning brain in the way that a factory chimney produces smoke. The problem is
that whereas everyone can see the smoke coming out of a factory chimney, I can only see the
consciousness being produced by my brain. There's no way of telling whether other people's brains
produce mental stuff too.

Here's the tricky part:

Imagine someone exactly like you, in every physical detail. Physically, you and your double couldn't
be told apart, even using a microscope. Call him 'Jon'. Jon lives on Twin Earth in a Solar system just
like this one on the far side of the Galaxy. The only difference is that Jon's brain merely accounts for
his speech and behaviour. Jon's brain does not produce any mental stuff. For Jon, all is darkness
within. Jon, in other words, is a perfectly disguised zombie.

But because Jon is exactly like you, and because Jon lives exactly the same life on Twin Earth, Jon
has submitted a question to Ask a Philosopher about the problem of solipsism. It seems that Jon is
just as troubled by the problem as you are, even though Jon doesn't have a mind! Whatever caused
him to ask his question is the same as what causedyou to ask your question!

Well, that was the first kind of solipsist. As I said, not a knock-down punch, but a points win. The
second kind of solipsist says, How do I know that there's a world at all, outside my own mind?
Everything in the universe exists only because I do, it exists for me.If I did not exist, then the universe
would not exist either. Other people are merely characters in the story of my world.

That's a far tougher proposition. I give an argument against this metaphysicalkind of solipsism in my
book Naive Metaphysics. It is possible to be a metaphysical solipsist, but at the price of giving up the
concept of truth.The trouble is, what do you say against someone who says that they are prepared to
pay that price?

Geoffrey Klempner