- From: Jim Hendler <hendler@cs.umd.edu>
- Date: Mon, 17 Nov 2003 21:23:49 -0500
- 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