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Re: Rules WG -- draft charter -- NAF

From: Stefan Decker <stefan@ISI.EDU>
Date: Tue, 18 Nov 2003 01:46:16 +0000
Message-Id: <>
To: Jim Hendler <hendler@cs.umd.edu>, Benjamin Grosof <bgrosof@mit.edu>, Sandro Hawke <sandro@w3.org>
Cc: adrianw@snet.net, www-rdf-rules@w3.org, phayes@ihmc.us

Please apologize my ignorance - what is hard about doing closed world 
reasoning on a giving  RDF graph?


At 01:38 AM 11/18/2003, Jim Hendler wrote:
>Ben - I think you miss my point - I didn't say figuring out a way to do 
>NAF would be a bad thing, I said it would be a very HARD thing, and one 
>for which there is no current de facto solution -- WOWG looked for a way 
>to do this, and realized we would not be able to do it -- I don't see why 
>the rules group would expect success unless they could start from an 
>existing solution -- and I've seen no proposal with a solution that seems 
>workable.  If it's going to be part of the charter, then I would want to 
>see at least 1 workable solution before the WG starts...
>  -JH
>p.s. WOWG's objective, which we didn't achieve, is mentioned in our 
>requirements [1]
>O3. Ability to state closed worlds     Due to the size and rate of change 
>on the Web, the closed-world assumption (which states that anything that 
>cannot be inferred is assumed to be false) is inappropriate. However, 
>there are many situations where closed-world information would be useful. 
>Therefore, the language must be able to state that a given ontology can be 
>regarded as complete. This would then sanction additional inferences to be 
>drawn from that ontology. The precise semantics of such a statement (and 
>the corresponding set of inferences) remains to be defined, but examples 
>might include assuming complete property information about individuals, 
>assuming completeness of class-membership, and assuming exhaustiveness of 
>subclasses.     Motivation: Shared ontologies, Inconsistency detection
>[1] http://www.w3.org/TR/webont-req/#section-objectives
>At 12:13 -0500 11/15/03, Benjamin Grosof wrote:
> ><x-flowed>Hi Jim and all,
> >
> >At 03:18 PM 11/14/2003 -0500, Jim Hendler wrote:
> >>Ben-
> >>  I agree w/Sandro - NAF requires identifying a set of facts it works over
> >> (the domain) - but RDF graphs,  but their very nature are open -- so what
> >> sound easy suddenly becomes very hard.  We attcked this problem in WebOnt
> >> (see our reqs document and issues lists - sorry, I'm on slow connection
> >> don't have the URIs, but they are one link from
> >> http://www.w3.org/2001/sw/WebOnt) - we wanted a way to have a local
> >> unique names assumption - but couldn't solve the problem -- I bet the
> >> local domain naming is at least as hard, probably harder
> >
> >Would you please send me specific links when you can? I looked at the OWL
> >requirements and issues list documents and I couldn't   easily figure out
> >which parts of them you were referring to.
> >
> >>  here's an example, tell me whaty you would do
> >>
> >>You say
> >>  Rule1 - if person(shoesize) != large then A
> >>  Rule2 - if person(shirtsize) != large then B
> >>  RULES-CLOSED-OVER http://www.foo.bar/document1.rdf
> >>
> >>and that seems fine,  but document1 includes
> >>   :Joe owl:class :person.
> >>   :Joe shoesize :large.
> >>   :Joe nickname "the gorilla".
> >>  :person rdf:type foo:human.
> >>
> >>now, foo is a namespace document which contains a bunch of facts about 
> humans.
> >>It is clear that A is false, because the document you're closed over says
> >>his shoesize is large
> >>But what about B being true?   We see that this document doesn't include
> >>that his shirtsize isn large, but what is on foo:?  Maybe it says anyone
> >>with the nickname "the gorilla" where's a large shirt, maybe it refers to
> >>another document, ad infinitum.
> >>  So when there is a web of graphs refering to terms in other graphs, etc
> >> - how do you know where things stop?  (see www-sw-meaning for a lot more
> >> dicussion of this issue!)
> >>  this is also only one simple manifestation of this problem -- when you
> >> talk about documents that are changing, scraped, etc. (all of which come
> >> up on the web) it gets even uglier
> >>
> >>  Sandro put it well - it's not that we cannot do NAF, it's that designing
> >> the mechanism for definining the bounds of a graph on the web is still an
> >> unsolved problem --
> >
> >Thanks for the example, it helps.
> >I think you've put your finger right on the nub of the problem.
> >I was indeed presuming that there is a mechanism to define the bounds of
> >the knowledge base / graph, i.e., to well-define the set of premises.
> >
> >>  if the rules group has to solve it to make progess, that is risky
> >> business....
> >
> >I think the Semantic Web needs to solve it in an initial fashion, and quite
> >soon.  There's a tremendous overambitiousness in thinking that this is
> >*not* critical path.  It's not so hard to do, either -- in the following
> >sense.  Programming languages "solved" it long ago with mechanisms that
> >check transitively for inclusion (such as the "make" facility in C).
> >The obvious approach is to just use that type of idea for the Semantic
> >Web.  Thus if the transitive closure of the "import" chains cannot be
> >determined and meet the usual criteria of well-definedness then there is a
> >KB scope violation of a "system-ish" nature.  This will force people to
> >define more carefully exactly which portions of other KB's that they are
> >importing -- including via more contentful module mechanisms within KB's --
> >and to do integrity checking on transitive closures of inclusion both
> >initially when KB's are developed and periodically/dynamically as KB's are
> >maintained/updated.
> >
> >  I know that some don't like the idea of having to do this.  I think the
> >alternative of not being allowed to define such scoping is, however,
> >extremely undesirable.  The idea of "all RDF anywhere on the web" as
> >something I would want to always *have to* use as my KB's scope is a
> >complete non-starter practically -- consider issues of data/knowledge
> >quality alone!  (I'm tempted to say it's ridiculous.  People talk about
> >"trust" on the Semantic Web.  The most basic mechanism for trust is simply
> >to know what set of premises the inferences were drawn from.  We'll be
> >laughed out of town in most practical IT settings if we don't have a good
> >story about this aspect of things.)
> >
> >If we take the approach I'm suggesting (and others have suggested it too)
> >then we don't have to get fancy about deep philosophy and unplumbed
> >territory of "social meaning", or wait for more research on "trust",  to
> >just get going on doing over the Web the kind of KR that has been proved in
> >useful in decades of practical applications (and for a number of years in
> >multi-agent systems).  We can then proceed incrementally/evolutionarily
> >over time, as we develop further use cases and techniques, to open things
> >up by having more implicit and relaxed mechanisms for importing / scoping
> >the KB's/graphs.   We should start with what we know works, in short, and
> >then work to improve upon it in the direction of reducing the burden of
> >defining inclusion/import scoping.  As a practical matter, if there is a KB
> >scope violation cf. above, then that doesn't mean we can't/won't do
> >inferencing, depending on the purpose and kind of inferencing -- some kind
> >of inferencing may be useful even when there is a violation.
> >
> >If we do it that way, we can have/do nonmon/NAF on the Semantic Web
> >essentially today, and develop additional techniques later for making the
> >scoping more flexible and convenient.
> >
> >
> >>  -JH
> >>p.s. Note that the OWL group rjected the solution that we could use the
> >>imports closure and define everything else as not included, because that
> >>would limit you to only those things defined in the DL profile, not all
> >>OWL and all RDF documents
> >
> >I'm confused by this.  "All OWL and all RDF documents" is way too big --
> >see above my comment about "all RDF on the Web".  When you say "DL profile"
> >I presume you mean the set of OWL imports statements.  What's the point of
> >an imports mechanism in OWL if everything else is included?  Perhaps I'm
> >not understanding what you're saying.
> >
> >In any event, the way to go is to define (a given KB as) importing of RDF
> >as well as OWL (and soon, more generally, semantic web rules knowledge base
> >modules as well), in the imports profile, and stick to the transitive
> >closure for most purposes.  Does that require extending the current imports
> >mechanism of OWL, e.g., to define a boundaried RDF graph as imported?
> >
> >>-- the rules language would have to face that same issue, but also deal
> >>with all things findable by Xquery ... yow!
> >
> >I don't see what XQuery has to do with it (at least not directly), if we're
> >talking RDF stuff.  XQuery is certainly related to Semantic Web Rules
> >(indeed, I was one of the first to press this point to the W3C team; back
> >in March 2001 I presented to them about it), but I don't see that Rules
> >"have to... deal with all things findable by XQuery".  More pertinent to
> >the main topic here is that XQuery deals quite ambitiously with very large
> >scale databases and as I understand it (from early versions I looked at)
> >has a well-defined boundary of what is queried over.  That's thus probably
> >further evidence towards the usefulness of my scoping suggestion about
> >imports closure.
> >
> >Benjamin
> >
> >>--
> >>Professor James 
> Hendler                   http://www.cs.umd.edu/users/hendler
> >>Director, Semantic Web and Agent Technologies     301-405-2696
> >>Maryland Information and Network Dynamics Lab.    301-405-6707 (Fax)
> >>Univ of Maryland, College Park, MD 20742          240-277-3388 (Cell)
> >
> >_________________________________________________________________________ 
> _______________________
> >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
> >
> ></x-flowed>
>Professor James Hendler                   http://www.cs.umd.edu/users/hendler
>Director, Semantic Web and Agent Technologies     301-405-2696
>Maryland Information and Network Dynamics Lab.    301-405-6707 (Fax)
>Univ of Maryland, College Park, MD 20742          240-277-3388 (Cell)

Received on Monday, 17 November 2003 20:46:29 UTC

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