X-Message-Number: 8626
Date: Fri, 26 Sep 1997 10:04:38 -0400
From: "John P. Pietrzak" <>
Subject: My brain is practically a Turing Machine
References: <>

Hi again, still catching up on stuff here.

Thomas Donaldson wrote:
> To John Pietrzak:
> 
> You're strictly correct that there is no such thing as a PRACTICAL
> Turing machine (Turing machines are hardly practical, being
> intellectual constructions only). I do hope, though, that you
> understood the point I was making when I made that statement.

Yes, in so far as the brain is a massively parallel computing device
(which, from overwhelming evidence, appears to be the case), real-world
parallel devices should have a significant advantage in simulating
mental activity.

> I will say the same about your ideas for using a Turing machine to
> simulate a brain. Sure. But they tell us nothing about simulating a
> brain on a computer in the real world.

Well, I wouldn't go that far. :)  The TM does for this case what it
does for any other problem in which it is used; it doesn't show how to
implement the solution to a particular problem, but it does _prove_
that, given the axioms are correct, the solution can be implemented
in some form on a processor.  This is significantly more than nothing
in my book.  But you've already noted this point further on in your
message.

> Whether or not anyone has defined one, I believe it would be useful to
> consider a device which is like a Turing machine but suffers from
> limits of various kinds: length of tape, speed with which it can write
> or read from its tape, and so on. Such a machine might allow us to
> find better bounds for the behavior of real machines at any given
> time, for instance. I will call such machines "practical Turing
> machines". They are yet another intellectual construction, and in the
> sense of existence that an idea has existence, I guess they must
> exist.

Unfortunately, the problem with this sort of construction is in holding
it to bounds which are appropriate to the real world.  Processors have
consistently gotten faster, smaller, cheaper, disk space gets larger
and cheaper.  New ideas (parallel processing, quantum computers, 
holographic storage, etc.) hold out promise of even greater increases
in the _rate_ of increase.  Of what value, then, would a mental model
with a particular bound in one of these variables be?  The new machine
would hold only until it's bounds were broken, and then anything proven
with it would be again subject to question.


John

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