John M writes:
> 1. do we have a REAL argument against solipsism?

Let me express how solipsism can be analyzed in the model where physical
reality is part of mathematical reality.

Let us adopt Bruno's UDA perspective: the Universal Dovetailer (UD)
is an abstract machine that runs all possible computer programs.
In this way it creates all possible universes, and more... it creates
all possible information objects: all of mathematics, logic, all written
texts, everything.  In particular it creates the information patterns
of conscious entities like you and me.

Let us assume that this in fact represents the reality of the multiverse,
that what we perceive and experience is all caused by the operation
of the UD, when it creates information patterns that correspond to
those experienes.  I know that many people here reject this hypothesis,
but let us follow it forward to see what it can say about solipsism.

The first thing to notice is that within the UD, each person exists more
than once.  There are many programs that include a particular information
pattern in their output, in fact an infinite number of programs.  Some of
those programs will look much like the kind of model a physicist might
construct for a "theory of everything".  It would include the physical
laws and initial conditions that define our universe.  Running that
program forward would create the entire history of our universe, including
the experiences of all of its inhabitants.

However there are other kinds of programs that would also create the
patterns of our conscious experiences.  Some might do it purely by random
chance: they might produce enormous outputs and somewhere buried in there
will be the pattern that corresponds to a portion of our experience.
Others would include bizarre universes such as one inhabited by aliens
who create computer simulations of other kinds of beings, and who have
created us.  Yet another example would be a universe composed only of one
person, with all that is outside of him being supplied by the computer
program, perhaps from some kind of table of sensory impressions, so that
only he is real within that universe.

Solipsism is the doctrine that only I exist, that everything else is an
illusion.  In the context of the platonic multiverse, it would correspond
to that last case: a portion of the UD program where only the one person
is in his universe, and nothing else in the universe is real.

So this raises the question: given that I exist multiple times within
the UD structure, and given that in some of them the universe I see
around me is real and in some of them it is an illusion, which is the
reality for me?  In which one do I actually exist?

I believe Bruno argues, and I agree, that this is a meaningless question.
You exist in all of them.  There is no single instance of your information
pattern which is "really you".  Your consciousness spans all of the
places in the UD where it is instantiated.  However, there is a related
question which is relevant: what will happen next?  If some of your
consciousness is in the real universe, and some of it is in universes
where you are an alien simulation, some in a universe where it is a
random fluctuation, and some in a universe where you are all there is,
how can you make a prediction about the future?  In the random universe
you would expect to disintegrate into chaos.  In the aliens, they might
open up the simulation and start talking to you.  In the solipsism case,
various bizarre things might happen.  And in the "plain vanilla" universe,
you would expect things to go along pretty much as you remember them.

Here is where I may depart from Bruno, although I am not sure.  I argue
that you can in fact set up a probability distribution over all of the
places in the UD where your mind exists, and it is based roughly on the
size of the part of the UD program that creates that information pattern.
Recall that the UD in effect runs all programs at once.  But some programs
are shorter than others.  I use the notion of algorithmic complexity
and the associated measure, which is called the Universal Distribution
(an unfortunate collision of the UD acronym).  Basically this says that
the measure of the output of a given UD program of n bits is 1/2^n.

This gives us a probability distribution over all the places our minds
are implemented, such that the shortest program(s) get the bulk of
the probability.  This has relationships to such traditional notions as
Occam's Razor, as Russell Standish has emphasized.  Just as we say that
the simplest explanation for our observations is likely to be correct,
so we can say that the simplest program which creates our experiences
is likely to be the one that governs what will happen next.

In principle, it should become possible eventually to turn this reasoning
into at least rough quantitative form.  We will eventually have a
complete model for our physical universe, so we can compute its measure
and determine how big a contribution that part of the UD makes to our
experiences.  Likewise we could create a model of the solipsist universe,
where only the person is real and all of his external experiences are
provided by the program.

It is very likely that it will turn out that the real-universe program
will be much smaller than the solipsist one.  The solipsist program has
to have an enormous table to supply all of the sensory experiences a
person will receive.  It also needs to have some mechanism to compute
his brain patterns, and probably the simplest will be to have an actual
physical universe model.  Building the brain patterns into an internal
table is another way to do it but will probably be even larger.  Any way
you look at it, the solipsist program is going to be enormous.

Reasoning in this way, we will not only be able to basically reject
the solipsist hypothesis, we can actually do so in a quantitative way!
We will be able to say, the fraction of your experiences due to the
solipsist universe, or equivalently the probability that it is true,
is this extremely tiny number.  And it will very likely be an incredibly
small number, 1/2^(10^20), perhaps.  (I wrote a long posting last year
which effectively estimated this number.)

This, then, is an example of the power of the assumption about the
platonic universe, that physical and mathematical reality are the same.
It not only sheds light on ancient and seemingly insolvable conundrums
such as solipsism, it should allow us to in principle produce quantitative
estimates of the role of solipsist universes in the larger reality.
If as I wrote yesterday we are able to eventually verify predictions of
this model in terms of physical observations, we would have achieved
a unification of physics and philosophy far deeper than has ever been
accomplished before.

Hal Finney

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