Diana asked:

How do we know that we really exist and we are not just someone else's dream or something like
that?

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"How do I know that I am not part of someone else's dream?" puts a peculiar twist on Descartes' story
of the Evil Demon.

In the First Meditation, Descartes imagines that there is an evil demon deceiving him into thinking that
he is awake, in contact with a world of physical objects around him, when in reality he is only
dreaming. Yet even if an evil demon deceives me, argues Descartes, there is a "me" being deceived.
Deceived about the existence of an external world or not, either way I must exist.

What would it mean to exist merely as part of another person's dream? At the end of Lewis Carroll's
Alice in WonderlandAlice asks her pet cat:

"Now Kitty, let's consider who dreamed it all. This is a serious question, my dear, and you should not
go on licking your paw like that— as if Dinah hadn't washed you this morning! You see Kitty, it must
have been either me or the Red King. He was part of my dream, of course — but then I was a part of
his dream too! Was it the Red King, Kitty? You were his wife, my dear, you ought to know — Oh Kitty,
do help to settle it! I'm sure your paw can wait!" But the provoking cat only started on the other paw,
and pretended it hadn't heard the question.

Carroll, or, rather Ludwig Dodgson, was fully aware of the absurdity of this question. If it was Kitty in
the role of the Red Queen, not Alice, who dreamed the dream about Wonderland, then Alice's
adventures existed only in Kitty's mind, not in the mind of Alice. Take away the real Alice — awake,
sound asleep or dreaming her own dreams, whatever they may be — and we are left with an 'Alice'
that is not self-aware, that does not see or hear or think, but merely appears, in Kitty's dream world, to
do such things.

We cannot accuse Descartes of overlooking the possibility of 'existing in someone else's dream' in
Lewis Carroll's sense, for it is not a scenario in which there is any trace of me,the subject. Yet it could
be argued that there is a possibility that Descartes does not consider. He assumes that as an existing
subject, in a dream world or a real world, I make judgements about my experiences, perform
inferences, ask questions, consider doubts. These are actions,albeit mental actions. But what if I did
not exist as a subject capable of actions, physical or mental? What if all these thoughts passing
through my mind are merely experiences being fed to me? This is a scenario that should not be
unfamiliar to readers of science fiction: the idea that one might exist inside the circuits of a
supercomputer as a character following the script generated by a virtual reality program.

I do not know whether that hypothesis ultimately makes sense — I suspect that it does not — but that
is the closest one can get to the idea that my reality as a conscious subject is illusory. In reality, there
is no "I" that sees, hears or thinks, even though there seems to be.

Geoffrey Klempner