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Re: Rules WG -- draft charter -- NAF

From: Benjamin Grosof <bgrosof@mit.edu>
Date: Thu, 13 Nov 2003 12:18:54 -0500
Message-Id: <5.1.0.14.2.20031113120343.02027548@po12.mit.edu>
To: Sandro Hawke <sandro@w3.org>
Cc: adrianw@snet.net, www-rdf-rules@w3.org, phayes@ihmc.us

Hi Sandro and all,

At 10:38 AM 11/13/2003 -0500, Sandro Hawke wrote:

>[ Pat Hayes added on the Cc:, in case he wants to correct my memory of
>what he agreed to, down in the last paragraph. ]
>
> > >      { ?game a Game.
> > >        NotKnown { ?game error ?e }.
> > >      }
> > >      =>
> > >      { ?game a PerfectGame }
> > >
> > >But... Well, at very least that's non-monotonic.   Any conclusion you
> > >reach like this MUST NOT be re-published, right?  Or, they must always
> > >carry with them the warning "if we assume my knowledge of the world is
> > >complete, then...."   And people wondering by and picking up this rule
> > >... well, do you have any reason to think they would know where to
> > >look for errors?   The naive user/software is almost certain to get
> > >false results from NAF rules like this.
> >
> > The approach taken in the declarative logic programs literature, and most
> > practical
> > rule reasoning applications, solves this in a quite simple fashion.
> > Every conclusion is drawn (and prescriptively sanctioned by the semantics)
> > relative to a **specified set of premises***.   In your terms, that set of
> > premises is "my knowledge of the world".
>
>But in traditional rule systems, a solid characterization of that set
>of premises is known by everyone who cares about the conclusions; in
>my Semantic Web scenarios, at least, it's not.

I think that that's the nub of the problem.  I don't think it's a necessary 
ambition
driven by use cases.  Any given application imports what knowledge
it wants to.  Then it's not "walking on water", it's just "walking" (cf. 
your below)


> > If a rule is written using NAF,
> > then the rules which depend on NAF should be viewed as belief policies.
> > KR is primarily about principles for reasoning with ***beliefs***, not
> > irrevocable truths.  (If you want irrevocable truth,
> > stick to mathematics and religion not practical daily living and its
> > pragmatic reasoning!)
>
>I don't see where the difference between belief and truth manifests
>here.  I plan to try adopting your suggestion here and see how it
>goes.
>
> > Your worries about dangers of usage are overblown.  There are decades of
> > successful applications usage
> > of NAF.  Probably hundreds of thousands of users have written Prolog, for
> > example.
>
>You're arguing that we'll be able to walk on water because many people
>have walked before.  How many of those prolog programs would still
>give only correst results if you randomly swapped clauses between
>them?  1% 2% 0.01% On the Semantic Web, they all have to.  Asserting a
>rule base which gives incorrect results when randomly combined with a
>rule base someone else has asserted (and with some clauses randomly
>dropped) makes one of you a liar.

Whoever does the combination takes responsibility for what
knowledge base premises they are asserting, cf. my
comments above.


> > This principle of relativity to a premise set is essential to any
> > nonmonotonic KR, not just
> > to NAF.
>
>Indeed.  Pat Hayes is pretty clearly on record as saying Semantic Web
>logics have to be monotonic.  And he's the logic guy who sat through
>all the RDF Core discussions about what RDF was really about.
>
>As I recall, after some hours of arguing with him at the March 2003
>DAML PI meeting (after your "rules debrief" presentation), you two
>came to consensus that he was right in terms of the web at large,

You seem to have gotten a misimpression.  No, we did not agree on that. We 
agreed on something else --
which will be a future paper hopefully...  --
my recollection is that roughly what Pat agreed to, and which I also 
believe even more strongly, is that if you have to deal with inconsistency 
arising during merging/importing of knowledge bases, then prioritized 
conflict handling is an important approach to pursue in research and may 
well turn out to be the least evil as something that one does/uses in 
practice, especially there are few major alternatives to it.   Perhaps Pat 
would like to clarify/amplify that? -- I'd rather let him speak for himself.

>but
>you were right that non-mon reasoning was okay for conclusions used
>only *within* an agent.

Yes we did agree on that.  I would more precisely replace "agent" here by 
"knowledge base", however.


At any given time, conclusions are used within one agent, in most current 
or envisioned application scenarios I have heard of.
That includes knowledge bases created by merging (including virtually, say 
by querying),
but then that covers most semantic web scenarios of *usage* of reasoning.
In those circumstances, in principle it is well defined what are the set of 
premises the agent is reasoning with, relative to which the conclusions 
(e.g., nonmonotonic) are then sanctioned.

>  Perhaps we need to revisit that agreement and
>characterize that line more precisely, if we're to agree on where NAF
>fits into the Rules charter.

Yup, trying to do that with this message...


>And as my previous mail said -- it's not like we can't do NAF/non-mon
>on the Semantic Web, we just have to explicitely name the premise
>set.  I gave two examples of how that might be done with little pain.
>
>     -- sandro

Best,
Benjamin


________________________________________________________________________________________________
Prof. Benjamin Grosof
Web Technologies for E-Commerce, Business Policies, E-Contracting, Rules, 
XML, Agents, Semantic Web Services
MIT Sloan School of Management, Information Technology group
http://ebusiness.mit.edu/bgrosof or http://www.mit.edu/~bgrosof
Received on Thursday, 13 November 2003 12:17:05 UTC

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