On 01 Nov 2012, at 05:27, meekerdb wrote:

On 10/31/2012 11:52 AM, Bruno Marchal wrote:

I don't see why denying mathematical realism would entail saying no to the doctor.

It implies not saying "yes" qua computatio. It implies NOT understanding what Church thesis is about, as to show it consistent you need the diagonalization, which use the excluded middle principle.
You can still say "yes", but only by using some magic.



The doctor isn't proposing to replace part of you brain with a piece of Platonia, he has a real physical device to implant.

This is not related. That will follow step 8.

Here, you have to be arithmetical realist to get an idea of what a computer is, and how it functions, as the physical one will approximate it, well enough, it is hoped.

Of course you can say "yes" to the doctor, just because you trust him. But comp is not "saying yes" to the doctor. Comp is the doctrine that saying yes will indeed work, once the artificial brain is a *computer*. The definition of computer makes no sense with arithmetical realism.

?? If I'm a materialist I could say yes because I think the artificial brain produces the same input/output signals.

But you need to be arithmetical realist to define what you mean by "same input-output". Arithmetical realism is not a big deal. It means that you believe that 2+2=5 OR 2+2≠5.





  I don't see why I would have believe in Platonia.

Comp use only arithmetical platonia, and that is just a poetical expression to say that you believe that 17 is prime independently of the existence of the Higgs boson.


I may believe that only some computations are instantiated and there are no infinities.

OK, but again, that is different. That's the move toward physical ultrafinitism. You can keep comp, up to step seven, and we are back on the fact that step 8 (the movie-graph argument) makes such move senseless.

But note that to just define the term computation, you need to be arithmetical realist. But if there were no step 8, indeed, you might have succeed in saving a form of materialism. I still miss what you don't understand in the step 8. you did not comment my recent answer on this. Maybe you could try to elaborate on your intuition. Why and how does a primitive matter change something in a computation or in the consciousness associated to it, and this in a Turing emulable manner?

Bruno


http://iridia.ulb.ac.be/~marchal/



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