Re: UDA revisited

From: Bruno Marchal <>
Date: Mon, 20 Nov 2006 14:35:50 +0100

Le 18-nov.-06, à 03:53, Russell Standish a écrit :
> I had a thought about an alternative way of expressing the UDA
> (universal dovetailer argument).
> Computationalism is the statement that "I am a computation".
I disagree. Well to be sure "disagreement is relative", and when I say
I disagree with you, it is not comparable with the disagreement with P.
Jones and apparently Brent Meker (who seems to discard the quantum MW
as well like if they were back at stage one on the everything
It would have been more correct to say "my life is a computation"
instead of "I am a computation". It could be misleading at some points.
Programs and machines are finite object (codable by natural numbers).
Computation are infinite (in general) and not all can be even just
associated to finite objects.
> To use
> the RITSIAR acronym, computations are real in the sense I am real. But
> the Church-Turing thesis gives a particular model of a computation, it
> is effectively defining a computation as something that there is a
> Turing machine emulating.
OK. With CT the "turing" adjective is no more necessary.
> The universal dovetailer is a computation. Contained within its
> execution trace, are the execution traces of all other programs,
> including itself. If we are a program, we can be found inside a
> universal dovetailer, which can be found within another UD (infinitely
> many, in fact).
Right, but the "finiteness" will make sense only from the 1 person
point of view. From the third person pov, it is important to keep in
mind the UD is finite and well defined, and general enough through the
> To say that there must be a physical computer on which the dovetailer
> should run, is rather similar to saying there must be an ultimate
> turtle upon which the world rests.
To say this you need induction, that is you need a theory of numbers
(like PA, ZF or even just RA).
But once you have RA, PA, or ZF, you do have an ultimate turtle!
True the UD will generate all possible UDs, but the "first one" is
unavoidable once you accept the notion of natural numbers together with
addition and multiplication. So why not take this into consideration?
> The little old lady was right in
> saying "its turtles all the way down". Of course it is also analogous
> to saying there must be a prime mover to start the causal chain. If
> God created the world, then it immediately poses the question "Who
> created God".
> Since it makes no difference in any observable respect whether we are
> living in a computer simulation running on a bare substrate, as one
> that is incidently computated as part of a universal dovetailer, or an
> infinite chain of dovetailers, we really can make use of Laplace's
> ripost to Napoleon "Sire, I have no need of that hypothesis" with
> respect to a concrete computer running our world.
OK, but note that this should be explained more precisely through the
1-3 pov distinctions.
> To rescue the primitive matter world, we need to deny the existence of
> the universal dovetailer. But this denies the Church Turing thesis -
> we have to say some computations exist (eg ourselves), but others
> don't. To make computationalism compatible with primitive materialism
> requires us to abandon the Church-Turing thesis and redefine what we
> mean by computation.
Actually this is exactly what people like David Deutsch would like to
do. They want to assume a physical world (for example obeying to QM),
and define computation is physical terms, for example by the notion of
"quantum computation".
But indeed this forces him to redefine "Church thesis" in naturalist
terms, and well, that is sort of revisionism, and it cannot work
because they have to postulate arithmetic (or the sinus function which
reintroduces arithmetic in analysis).
Up to those (perhaps minor at this stage) points I agree with you,
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Received on Mon Nov 20 2006 - 08:36:27 PST

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