From: Deborah McGuinness (dlm@ksl.stanford.edu)
Date: 10/24/01
Sandro provided the RDF way in his response below (which i agree is better). The typical way to handle getting existentials into DLs is to do so with cardinality (qualified or non-qualified) restritions. I just didnt know the RDF way so in my posting, i used the standard description logic way. d Date: Wed, 24 Oct 2001 15:07:10 -0400 From: Sandro Hawke <sandro@w3.org> There is a much simpler way, since RDF includes existential variables. <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns:example="http://example.com#"> <rdf:Description> <example:color rdf:resource="http://example.com#red"/> </rdf:Description> </rdf:RDF> This is parsed by the validator [1] to _:j70241 <http://example.com#color> <http://example.com#red> . As Pat's model theory [2] (and his various e-mails to me on www-rdf-rules) make clear, the "unlabeled" nodes (like _:j70241) should be interpretted as existential variables. This clarification to RDF is fairly recent, and not yet official, (it's just a working draft), but I haven't heard anyone objecting yet. I have heard several members of the original RDF group say this is what they meant. -- sandro [1] http://www.w3.org/RDF/Validator/ [2] http://www.w3.org/TR/rdf-mt/ Richard Fikes wrote: > > > >The knowledge base contains the statements: "Pat's car is blue, and > > > >there is something colored red." Somewhat more formally: > > > > > > > > RDF(PatsCar, color, blue). > > > > exists x (RDF(x, color, red)). > > > > > > Sorry to be dense, but how does one state "there is something colored > > > red" in DAML+OIL? > > > > > > Richard > > Your reply suggests that there is no direct of way of saying "exists x > RDF(x color red)" in a DAML+OIL KB, which is what I suspected. Since > the issue came up in a discussion of what statements in a DAML+OIL KB > would match a query of the form "RDF(?x, color, ?y)", it is central to > the discussion. > > Am I correct in concluding that there is no way of including an > existential variable as the subject or object in an RDF statement in a > DAML+OIL KB? (I suspect there is a way, but I don't see it.) > > Richard > > > there are a few ways, arguably, none perfect. > > > > the most direct is mincardinalityQ > > state that something has mincardinality 1 (or more) > > and the Q in this case is RED-THING > > > > (the bad thing about this is it requires a property to be defined for the > > mincardinalityq statement > > say MySpecialProperty so that i can say > > > > <daml:Restriction daml:maxCardinalityQ="1"> > > <daml:onProperty rdf:resource="#MySpecialProperty"/> > > <daml:hasClassQ rdf:resource="#RedThing"/> > > </daml:Restriction> > > > > (and this of course assumes red things to be defined which could be: > > > > <daml:Class rdf:ID="RedThing"> > > <daml:sameClassAs> > > <daml:Restriction> > > <daml:onProperty rdf:resource="#hasColor"/> > > <daml:hasValue rdf:resource="#red"/> > > </daml:Restriction> > > </daml:sameClassAs> > > </daml:Class> > > > > > > one could also make individuals (just for the purposes of implying a > > red-thing) > > and say foo has min cardinality 1 on a property MySpecialProperty and > > then have the value restriction on MySpecialProperty be RedThing and the > > filler of MySpecialProperty > > on foo, be bar. > > this implies that bar is red and thus, there exists a red thing. One > > would not need to explicitly create bar for the inference to be implied. -- Deborah L. McGuinness Knowledge Systems Laboratory Gates Computer Science Building, 2A Room 241 Stanford University, Stanford, CA 94305-9020 email: dlm@ksl.stanford.edu URL: http://ksl.stanford.edu/people/dlm/index.html (voice) 650 723 9770 (stanford fax) 650 725 5850 (computer fax) 801 705 0941
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