X-Message-Number: 14953
Date: Sat, 18 Nov 2000 21:18:50 -0500
From: Thomas Donaldson <>
Subject: more on machines and brains

Hi everybody!

I note the reply of Joseph Kehoe to my question about whether brains can
really be assumed to be computers. My problem is not so much with what
he said as his justification for it: several issues need to be remembered.

1. The changing connections of our neurons are not arbitrary. (Remember
   that each neuron is a small computer). The same may be said of the
   use of any newly created neurons. Both result from the work a brain
   is doing AT THAT TIME. 

   This raises some problems which bear on any proof that the machine
   counts as a Turing machine. The problems come from creation of 
   NEW tapes as distinct from using farther and farther away parts of
   a single tape. That the machine is working in real time adds an
   important feature to this: if the space on the single tape is
   far enough away, it cannot reach it in time to do the needed
   calculations soon enough. Creation of new tapes does not have
   this problem.

2. The possibility of imitating an entire brain with a single processor
   still raises problems. A variety of our neurons actually respond to
   sensory input; many more are processing sensory input. If we had 
   only one processor (with lots more circuitry for memory etc) the
   possibility of missing sensory input still remains; the same problem
   happens with PROCESSING sensory input. We can't just look the other
   way while we're doing something else.

   As I said originally, the fact that our brains work in real time is
   not just a minor issue, but important for understanding them. 

I would like Joseph Kehoe (or anyone else) to give a much moredetailed
answer, quoting results from books or papers, on this question. I am
willing to be quite patient while waiting for that answer. Over and 
above any answers to me, I understand that some computer scientists 
have imagined machines with special features (say the ability
to compute with real numbers) which makes them not Turing at all. 
Though our brains aren't primarily designed by evolution for computing
with numbers, such possibilities need considering. 

In terms of actual results in the literature, support for brains as
Turing machines requires not just that a machine with many "neurons"
can be reduced to a machine with one, but that machines with varying
numbers CREATED IN RESPONSE TO A NEED FOR A COMPUTATION can be
reduced to just one. The simple fact that brains are also real-time
machines also must be dealt with: at some point a large enough number
of neurons will create an object which cannot be imitated by any 
single processor IN PRINCIPLE, for the simple reason that no single
processor could do the required work fast enough. The fact that we're
processing our sensory input makes a difference here.

AS I've said before, but wish to emphasize here, I am not arguing that
a machine imitating human beings enough to be conscious etc just like
us is impossible. I am raising the question of whether or not such a
machine is a TURING MACHINE.


		Best wishes and long long life to all,

			Thomas Donaldson

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