W3C home > Mailing lists > Public > www-rdf-rules@w3.org > November 2003

Re: Rules WG -- draft charter -- NAF

From: Jim Hendler <hendler@cs.umd.edu>
Date: Mon, 17 Nov 2003 21:23:49 -0500
Message-Id: <p05200f5fbbdf30ee97f4@[10.0.1.5]>
To: Stefan Decker <stefan@ISI.EDU>, Benjamin Grosof <bgrosof@mit.edu>, Sandro Hawke <sandro@w3.org>
Cc: adrianw@snet.net, www-rdf-rules@w3.org, phayes@ihmc.us

At 1:46 +0000 11/18/03, Stefan Decker wrote:
><x-flowed>Please apologize my ignorance - what is hard about doing 
>closed world
>reasoning on a giving  RDF graph?
>
>Best,
>          Stefan
>
>

Tell me how you define what a given RDF graph is on the WWW --  if 
all the facts are on a single document, then it isn't too hard - but 
if it is linked to other documents (other than the RDF namespace) it 
gets hard.  RDF is designed to be open, merged, and able to reference 
other RDF -- those are its main features -- but once in a graph form, 
and using pointers elsewhere, then you get a lot of issues that need 
to be resolved -- in an earlier message I gave a simple example -- 
here's another one -- suppose I point you to the Foafbot results and 
claim they are closed -- but then tomorrow a new scrape is made and 
some new stuff is there, is that the same closed document?  This 
could be solved using some sort of timeouts and etc - but how do you 
do that?  Without a normative way of handling time, how do you 
represent the graph being closed at time T?
  In short, I repeat, there's nothing unsolvable about doing this -- 
but it is not as obvious as it appears, and without some sort of 
solutions already offered, I worry it is premature to try to 
standardize.



>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)
>>
>
>
>
>--
>http://www.isi.edu/~stefan
>
>
></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 21:23:46 UTC

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