Re: Constraints on "everything existing"

From: Jean-Michel Veuillen <>
Date: Thu, 23 Jan 2003 15:08:00 +0100

My comments at the bottom too.

At 08:51 AM 1/22/2003 -0800, Eric Hawthorne wrote:
>My comment at the bottom of the message.
> Eric
>Jean-Michel Veuillen wrote:
>>>Eric Hawthorne wrote:
>>>Unless a world (i.e. a sequence of information state changes)
>>>has produced intelligent observers though, there will be
>>>no one around in it to argue whether it exists or not.
>>Then our universe did not exist before there were intelligent observers
>>in it,
>>which is not true.
>>I think that is better to say that all self-consistent mathematical
>>structures exist.
>>To restrict existence to universes containing SASs (self-aware structures)
>>is not only is very cumbersome but leads to contradictions.
>Perhaps we're just quibbling about terminology.
>My argument for a narrower definition of "exists" would be that
>if everything (or even just "everything self consistent") exists, then
>perhaps existence in that sense is not that interesting a concept.
>So I posit that a better definition of "exists" or "classically exists"
>is: "self-consistent, and metric and organized to the degree to be observable"
>Notice that this does not require "is observed". It requires "would
>be observable if observers happened to be around." So our Earth 3 billion
>years ago was still "observable" in this sense, even though we weren't
>there yet.
>So, in otherwords, I define "exists" as
>"that which is an aspect of a structure which is of the form/behaviour as to
>be, in principle, observable".
>I think we will be able to define a set of properties (stronger than just
>self-consitency) that will define "in principle, observable". <--
>difficult exercise.
>All other "self-consistent mathematical structures" are, to me, just
>"potentially or
>partially existent", because there is something wrong with their properties
>that would make them, in principle, unobservable.
>Vague statement building up this intuition:
>The operative question is whether a mathematical structure can only be
>"abstract" (without observable instantiation) or whether it can also be
>I would argue that these other less-than-existent
>"self-consistent mathematical structures" may be part of "quantum
>but can never be part of an existent world that exhibits classical physical

I agree that "in principle, observable" is difficult to define. If, instead
of looking
at the Earth 3 billion years ago, you looked at our universe 1 second after the
Big Bang, would you say that it was "in principle, observable" ?
   Even if you managed to show that the answer should be Yes, you would then
have to show that for another universe whose parameters would differ by only
the slightest amounts, the answer should be No. One of these universes would
exist one second after the Big Bang, not the other one, which would be very
much against intuition.
   And if you answer No, you have to cope with the fact the answer is now Yes,
so you would have our universe which did not exist one second after the Big
and which exists now.

Here is what I would propose:

Following David Lewis and Modal Realism
all possible worlds exist.

Max Tegmark proposes in
that only worlds which have mathematical existence exists. This gives
the self-consistent constraint. I agree with that.

Tegmark then goes on and gives an "operational definition" of existence
which requires SAS's. I think that this unfortunate: If we were the only
SAS's in this universe and blew up the Earth, our universe would not
cease to exist. If we agreed that it did and if SAS's appeared somewhere
else later, would they say that they universe did not exist, then existed
when it was inhabited by us, then did not exist and then existed again ?
I find it absurd to make the existence of an universe depend on the fact
that it contains SAS's or not.

I simply propose that we say that some universes contain SAS's
and some do not, without any consequence on the existence of
these universes.

Jürgen Schmidhuber proposes that possible universe means
computable universe (without any reference to SAS's). See:

Again, I think it is simpler to say that some possible universes
are computable, and that some are not, and that what Schmidhuber
says in his article applies to computable universes only.

Received on Thu Jan 23 2003 - 09:09:48 PST

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