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RE: Rules WG -- draft charter -- NAF

From: Minsu Jang <minsu@etri.re.kr>
Date: Thu, 13 Nov 2003 16:50:30 +0900
To: "'Sandro Hawke'" <sandro@w3.org>, <adrianw@snet.net>
Cc: <www-rdf-rules@w3.org>
Message-ID: <000001c3a9ba$cf85f5e0$8c4bfe81@ZEBEHN>

I think NAF is still useful when it comes to the web. 

The 'perfect game' rule Sandro proposed expresses very subjective
knowledge. Someone could think of a game as perfect if she could not
any error report on it. But some other person may not think like that.
We use NAF-kind of reasoning everyday.

And writing a rule is very different from actually deriving conclusions
from it.
You can reuse the rule in its represented form. Get the rule and run it
your system. Then, you can choose as many facts to be fed to the rule 
as you want from any ontologies you want. The rule is the same, but 
the conclusions derived from it would be very different depending on 
who uses it. The importance of rule lies not only in its representation,

but also in its running environment.

At the representation level, I think NAF is a very useful knowledge
representation element (or a reasoning tactic), even for the web.

Minsu Jang

> [Sandro wrote,]
> I think it hinges on a basic point: being a Semantic Web rule language
> means more than just being a "standard" rule language.  If the goal
> were just to make a common language that Prolog, OPS-5, and SQL
> systems could use, then where would the Web come in?  No, the point I
> think is to allow everyone to publish their rules as reusable data, as
> assertions of one's belief about the world.  Others can then wander by
> a pick up a random rule or two as being useful.  (Whether they chose
> to believe it is a separate matter.)
> This makes NAF rather more tricky, I think, than in traditional rule
> systems.  This is the motivation for calling it out-of-scope.  Let's
> see....  Imagine the rule "If a game has no errors, it's a perfect
> game."
>      { ?game a Game.
>        Not { ?game error ?e }.
>      }
>      =>
>      { ?game a PerfectGame }
> But, as you say, this means someone has to SAY the game has no errors,
> they can't just not list any errors and still have the conclusion be
> supported.    So you want something more like:
>      { ?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.
> So what can you do?  W3C SWAD (mostly TimBL) has been looking at this
> for a while, and Cwm implements two solutions via built-ins [2]:
>     log:includes + log:notIncludes statements depend on whether some
>     web-accessible RDF graph includes some subgraph.  This lets you
>     say the game has no errors if http://foo.bar doesn't list any
>     errors.
>     log:definitiveDocument approaches this from the other side,
>     letting you say http://foo.bar lists all "errors", by the very
>     definition of "errors".   An exported SQL database with usual
>     closed-world semantics can simply include that it's a
>     log:definitiveDocument for all its predicates/columns, and the
>     expected NAF-like behavior should ensue.
> I'm sure there are other approaches, and I don't think the performance
> issues are well-understood yet, but it makes me think something is
> doable.   It also reminds me that the rule language here, even if very
> like Prolog, etc, in some ways, also needs to be quite different to
> be good for the Semantic Web.
>        -- sandro
> [1] 
> http://lists.w3.org/Archives/Public/www-rdf-rules/2003Nov/att-
[2] http://www.w3.org/2000/10/swap/doc/Reach

Minsu Jang
Senior Member of Engineering Staff
Business Knowledge Research Team
Electronics and Telecommunications Research Institute
Phone: +82-42-860-1250 Fax: +82-42-860-6790 
Received on Thursday, 13 November 2003 02:50:40 UTC

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