From: Harold Boley (boley@informatik.uni-kl.de)
Date: 11/12/02
pat hayes wrote: ... > >Looking at the RDF Model Theory edited by Pat, we are still open to various > >possibilities for "simply indicating the existence of a thing, without > >using, > >or saying anything about, the name of that thing" > >(http://www.w3.org/TR/rdf-mt/#unlabel). > > > >It now is clear to me that anonymous logical variables are no solution, > >since > >they are universally, not existentially, interpreted in facts. > > Doesnt that depend on the logic you are using? I was talking about rules and facts in Horn logic. To give an example in pure Prolog syntax, instead of the ground fact has-creator("http://www.w3.org/Home/Lassila","Ora Lassila"). someone could assert the non-ground fact (using "_" as the anonymous logical variable) has-creator(_,"Ora Lassila"). This is equivalent to the non-ground fact (using "X" as a named logical variable that is fresh in this clause) has-creator(X,"Ora Lassila"). X is interpreted universally, not existentially. So, anonymous logical variables in Prolog facts are unrelated to anonymous nodes (bNodes) in RDF triples. In Prolog queries, however, anonymous logical variables are interpreted existentially. > >I now think RDF's bNodes are more akin to some of the many uses of null > >values in > >relational databases or, perhaps, closer to RDF, in object-relational > >databases. > >A quick Google search revealed a related remark in "Topic Maps and RDF" > >(http://www.isotopicmaps.org/pipermail/sc34wg3/2002-August/000495.html). > > > >Null values are absent from Prolog, but in Dagstuhl discussions with Jens > >Dietrich > >(http://www.jbdietrich.de/about.html) we thought they would be important in > >RuleML > >anyway (he is working on RuleML extensions for relational databases). > > Harold, please, calm yourself. They are just existential variables. > Logic has had them since about 1878. Yes, I started with this at the beginning and came back to this at the end: It's the most logical interpretation of RDF's bNodes. But I think the excursion into relational databases is a worthwhile one, if only because some RDF triple stores have been interfaced to, or implemented in, relational databases. I guess that you could generalize your RDF analysis to relational databases and (re?)discover that some of the many uses of null values are just existential variables. > >Yet, this would not give us the local existential scope of RDF's bNodes. > >We hope our analysis of the RDF Model Theory will lead to a good solution > >for > >RuleML bNodes as well. > > > >Perhaps we should also have a closer look at eigenvariables in Lambda-Prolog > >(http://www.cse.psu.edu/~dale/lProlog/). > >Maybe we can even learn something here from the discussion of the "line of > >identity" > >in Charles Sanders Peirce's Existential Graphs > >(http://users.bestweb.net/~sowa/peirce/ms514.htm). > > Lets not get crazy here. There isn't anything very deep or mysterious > about existential quantifiers. No, but perhaps in their diverse graphical representations. Since RDF's graph models are so important to the RDF and Semantic Web community, perhaps it would be worthwhile to still try to optimize these graphs a bit. > Pat > >Ultimately, we may need explicit (existential) quantifiers for glueing > >together > >a conjunction (or a rulebase) of facts. To be specific, we may need rulebases like these: ... normal facts and rules ... (Ex X has-creator(X,"Ora Lassila"). was-accessed-by(X,"Ora Lassila"). ) ... normal facts and rules ... Harold
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