- From: Jos de Bruijn <jos.debruijn@deri.org>
- Date: Wed, 22 Jun 2005 19:18:20 +0200
- To: Jim Hendler <hendler@cs.umd.edu>
- CC: dreer@fh-furtwangen.de, public-sws-ig@w3.org
-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Jim Hendler wrote: > Jos- > that's not quite right as I understand it - to use an example from the paper I cited - > > Given an ontology containing only a single RDF triple: > <#pat> <#knows> <#jo>. > the answer to a query asking if pat knows exactly one person would be "no" under > RDF's open world semantics, but "yes" under the closed world semantics of Datalog. > > there's no negation explicit here. If you would write down the query you mentioned, you would see that the query includes negation. Fact is that a First-order theory: knows(pat,jo) entails only the fact: knows(pat,jo) A Datalog program: knows(pat,jo) entails only the fact: knows(pat,jo) > > btw, this is not a small issue to me -- without some way to control default reasoning (i.e. limit the scope of axioms and facts that are considered for a set of queries) there is just no way to link things reliably on the Web (and without the linking, why bother with Web). > As far as I'm concerned, the minute one assumes there is a mechanism to close a graph (and several are floating around), then ontology and rules go together just fine and lots of the layering is fixed. On the other hand, if a rule can be stated in general that has a negation as failure then we have a problem. > In the old days, for example my thesis work, we used "ThNOT" in langauges like MicroPlanner, NASL, etc. and that was some of the earliest work on this stuff. The idea was you could use a rule like this > P :- ThNOT Q. > to mean P is true if you cannot prove Q. Implicit in this, and many of the systems since, was the idea that there existed "A knowledge base" which was all the facts that this rule could be used on. > On the Web, I would want this to read something like > P :- (Thnot Q) given R. > where "given R" somehow means with respect to some database, some graph, or some other nameable and definable entity. > Thus, for example, one could imagine running a query against some graph (and note in theory that this could be the identity query, meaning it is equal to everything in the graph) and then stating that some set of rules is applied with respect to the resulting graph. These rules having NAF would be great and good and wonderful. If I applied your rules to a different graph, it would be up to me to decide if I wanted to restrict things in my graph to control the scope of the application of your rules -- this sounds like a powerful and crucial mechanism, and with it a lot could be done. Without it, rules on the web seem to me to either be just doing logic programming with a consistent syntax (in which case why invent all these languages, let's just use Prolog on the Web) or pretending the whole Web can be treated like a closed world - and that's, in my mind, where the dragons are... It indeed seems like a good idea to define scoped default negation. We did not address this issue in WRL, but it would be a good topic to address for the W3C rules working group (if it's going to be formed). Best, Jos > JH > > > > At 10:46 -0400 6/22/05, Jim Hendler wrote: > >At 12:04 +0200 6/22/05, Jos de Bruijn wrote: > > > >-- > >Professor James Hendler Director > >Joint Institute for Knowledge Discovery 301-405-2696 > >UMIACS, Univ of Maryland 301-314-9734 (Fax) > >College Park, MD 20742 http://www.cs.umd.edu/users/hendler > >-- > > Professor James Hendler Director > Joint Institute for Knowledge Discovery 301-405-2696 > UMIACS, Univ of Maryland 301-314-9734 (Fax) > College Park, MD 20742 http://www.cs.umd.edu/users/hendler - -- Jos de Bruijn, http://www.uibk.ac.at/~c703239/ +43 512 507 6475 jos.debruijn@deri.org DERI http://www.deri.org/ - ---------------------------------------------- Only two things are infinite, the universe and human stupidity, and I'm not sure about the former. -- Albert Einstein -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.2.5 (GNU/Linux) Comment: Using GnuPG with Thunderbird - http://enigmail.mozdev.org iD8DBQFCuZ1b4lqeiwiiHN4RAhJvAJ948dnymE2+c/m1nyjzZqHefZisOwCfVYK4 KZVSfp82ngr1wnDRpdWeZY4= =hEoG -----END PGP SIGNATURE-----
Received on Wednesday, 22 June 2005 17:18:32 UTC