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From: smoliar@vaxa.isi.edu (Stephen Smoliar)
Newsgroups: comp.ai
Subject: Re: simulating brains
Message-ID: <15299@venera.isi.edu>
Date: 13 Oct 90 17:36:13 GMT
References: <1990Oct2.221006.3024@csc.anu.oz.au> <15631@csli.Stanford.EDU>
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Reply-To: smoliar@vaxa.isi.edu (Stephen Smoliar)
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Lines: 44

In article <15631@csli.Stanford.EDU> weyand@csli.Stanford.EDU (Chris Weyand)
writes:
>In <1990Oct2.221006.3024@csc.anu.oz.au> ada612@csc.anu.oz.au writes:
>
>>The sense of my final parenthesis is that I find the standard idealizations
>>of computability theory to be a very dubious framework for thinking
>>about brains.  Computability theory is about what can be done eventually,
>>whereas brains have to keep up with the real world, always providing some
>>sort of output in response to the current input.
>
>So don't use computatbility theory.  I mean there really is a difference
>between computers such as the MacIIcx I'm using right now and Turing Machines.
>The main one being that TM's can't be fitted with an array of sensors and
>effectors or anything else physical since they themselves are not.  Also, sure
>computatbility theory talks about what functions can be computed and is not
>concerned with time or space efficiency.  But we who program real computers
>are very concerned with those issues.  Just because the theory says little
>about efficiency doesn't mean that computations can't be done efficiently.
>
Efficiency is only part of the story.  More important is that computability
theory is concerned with FUNCTIONS, in the strict mathematical sense of the
word, which is to say a relation which associates with every element from some
domain space at most one element from a range space.  Within this theory
computation is a finite process which eventually halts given any domain
element for which such an association exists.  However, there are plenty
of things that computers do which cannot be reduced to such functions.
For example, operating systems do not halt in a finite amount of time
and return a function value (at least they are not designed to do so).
If we want to talk about simulating a brain, we would do better to consider
an operating system as an appropriate metaphor than a function which computes
a polynomial.  Computability theory may then tell us about certain functions
which we may want to build as COMPONENTS of such a system, but that does not
guarantee that we shall gain any insights about building the whole system.

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