From newshub.ccs.yorku.ca!ists!torn.onet.on.ca!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!mips!darwin.sura.net!cs.ucf.edu!news Mon Jun 15 16:04:43 EDT 1992
Article 6212 of comp.ai.philosophy:
Newsgroups: comp.ai.philosophy
Path: newshub.ccs.yorku.ca!ists!torn.onet.on.ca!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!mips!darwin.sura.net!cs.ucf.edu!news
>From: clarke@acme.ucf.edu (Thomas Clarke)
Subject: Re: Physical versus Computaional (was Re: Transducers)
Message-ID: <1992Jun11.172846.9370@cs.ucf.edu>
Sender: news@cs.ucf.edu (News system)
Organization: University of Central Florida
References: <1992Jun11.153432.4670@mp.cs.niu.edu>
Date: Thu, 11 Jun 1992 17:28:46 GMT

In article <1992Jun11.153432.4670@mp.cs.niu.edu> rickert@mp.cs.niu.edu (Neil  
Rickert) writes:
> In article <1992Jun11.132823.7139@cs.ucf.edu> clarke@acme.ucf.edu (Thomas  
Clarke) writes:
>   (in reply to Neil Rickert <4138.708217481@mp.cs.niu.edu>)  
> 
> >You are using "physical" in a much different way than I would.

>   Compare this to thought.  There are things that you can think about that
> will make you so sad tears will show up in your eyes.  Those tears are
> physical.  They indicate the occurrence of physical actions in the body
> beyond the transfer of neural signals.  The question of how central these
> physical actions are to thought remains controversial.
> ....
>   You are right.  I am using "physical" in a different way from your
> usage.  Your usage of the terms just does not make sense to me.

As I suspected, "What we have here is a failure to communicate."
According to my trusty on-line Webster, I am using physical:
	2b (2): (2): characterized or produced by the forces and operations 
               of physics
I think you are using physical:
	3a: of or relating to the body

Thus, is it correct to say that when you "agree with Harnad's assertion 
that thought (or at least human thought) is physical", that you
mean that bodily responses are somehow involved in thought processes?
Actually, I substantially agree with this statement, but I also
use physical in the other sense with respect to the brain; see below.

> >My use of physical versus computational in the context of the brain
> >would be to distinguish between the analog of neural net implementation 
> >of x*y versus a digital implemenation of x*y.
> 
>   Excuse me, but neural net implementations usually run on digital
> computers.

If I may elaborate a bit, not all neural nets run on digital computers;
brains use something like spiking rates as analog variables, and
Carver Meade's chips use voltages as variables.  Thus, the point of
my little example was to show that there are two ways for such a
"physically implemented" neural network to compute:  directly
using analog variables, and indirectly via digital techniques. 
Digital machines (Turing, von Neuman, whatever) can only compute
digitally.  [I know very few agree with me about its significance
for cognition, but analog computation opens the door for physical
effects arising from quantum mechanics, non-linear chaotic dynamics
etc to enter the brain.]

If I understand you correctly, you view the brain as an essentially
computational organ embedded in the body.  Futhermore, the 
computations carried out by the brain can be well approximated by
the computations of a digital machine.  

I think of the functioning of the brain is still poorly understood.  
It is probably a result of my mathematical background, but I suspect that  
continuum mathematics  (e.g. differential geometry, ???) must play 
a role in its explanation, mathematics that is difficult to 
approximate on a digital machine. 
--
Thomas Clarke
Institute for Simulation and Training, University of Central FL
12424 Research Parkway, Suite 300, Orlando, FL 32826
(407)658-5030, FAX: (407)658-5059, clarke@acme.ucf.edu


