Russell Standish writes:

> I had a thought about an alternative way of expressing the UDA
> (universal dovetailer argument).
> Computationalism is the statement that "I am a computation". 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. 
> 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).
> 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. 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.
> 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.

But if a physical universe is needed to run the UD, without a physical universe 
there is no UD. It's a circular argument unless you have some other argument 
showing a computation can run without physical hardware.

Stathis Papaioannou
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