Path: utzoo!attcan!uunet!samsung!rex!wuarchive!udel!princeton!phoenix!harnad
From: harnad@phoenix.Princeton.EDU (S. R. Harnad)
Newsgroups: comp.ai
Subject: Re: What is a Symbol System?
Summary: Implicit vs. Explicit Rules and Representations
Keywords: computation, symbol manipulation, syntax, formality
Message-ID: <11657@phoenix.Princeton.EDU>
Date: 21 Nov 89 01:06:33 GMT
References: <11640@phoenix.Princeton.EDU> <17189@netnews.upenn.edu>
Organization: Princeton University, NJ
Lines: 61



Dan Hardt hardt@linc.cis.upenn.edu University of Pennsylvania wrote:

> I'm not sure how you can sharply distinguish between a system
> that is interpretable as rule-governed and one that is
> explicitly rule governed. Perhaps you have in mind a connectionist
> network on the one hand, where what is syntactically represented might
> be things like weights of connections, and the rules only emerge from the
> overall behavior of the system; on the other hand, an expert system,
> where the rules are all explicitly written in some logical notation.
> Would you characterize the connectionist network as only interpretable
> as being rule-governed, and the expert system as being explicitly 
> rule governed?  If it is that sort of distinction you have in mind,
> I'm not sure how the criteria given allow you to make it.  If fact, I
> wonder how you can rule out any turing machine.

I'm willing to let the chips fall where they may. All I'm trying
to do is settle on criteria for what does and does not count as
symbol, symbol system, symbol manipulation.

Here is an easy example. I think it contains all the essentials:
We have two Rube Goldberg devices, both beginning with a string
you pull, and both ending with a hammer that smashes a piece of
china. Whenever you pull the string, the china gets smashed by the
hammer in both systems. The question is: Given that they can both be
described as conforming to the rule "If the string is pulled, smash the
china," is this rule explicitly represented in both systems?

Let's look at them more closely: One turns out to be pure causal
throughput: The string is attached to the hammer, which is poised like
a lever. Pull the string and the hammer goes down. Bang!

 In the other
system the string actuates a transducer which sends a data bit to a
computer program capable of controlling a variety of devices all over the
country. Some of its input can come from strings at other locations, some
from airline reservations, some from missile control
systems. Someone has written a lot of flexible code. Among the
primitives of the system are symbol tokens such as STRING, ROPE, CABLE,
PULL, HAMMER, TICKET, BOMB, LOWER, LAUNCH, etc. In particular, one
symbol string is "IF PULL STRING(I), LOWER HAMMER(J)," and this sends
out a data bit that triggers and effector that brings the hammer down.
Bang! The system also represents "If PULL STRING(J), LOWER HAMMER(J),"
"IF PULL STRING(J), RELEASE MISSILE(K)," etc. etc. The elements can be
recombined as you would expected, based on a gloss of their meanings,
and the overall interpretation of what they stand for is systematically
sustained. (Not all possible symbol combinations are enabled,
necessarily, but they all make systematic sense.) The explicitness of
rules and representations is based on this combinatory semantics.

It is in the latter kind of symbol economy that the rule is said to
be explicitly represented. The criteria I listed do allow me to make this
distinction. And I'm certainly not interested in ruling out a Turing
Machine -- the symbol system par excellence. The extent to which
connectionist networks can and do represent rules explicitly is still
unsettled.
-- 
Stevan Harnad  Department of Psychology  Princeton University
harnad@confidence.princeton.edu       srh@flash.bellcore.com
harnad@elbereth.rutgers.edu    harnad@pucc.bitnet    (609)-921-7771