From: pat hayes ([email protected])
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. As I say, that interpretation depends on the logical rules one is using. However, it is usual to interpret free variables as universals in a 'rule' language, I agree. >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. Right, the query always inverts. Gentzen was a clever old fellow. > >> >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. Sigh. The chief argument often made for the RDF graph syntax is that it is optimal for software to use. > >> 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 ... Right. I am a bit at a loss to know what the fuss is about. The actual logic involved here seems completely obvious. RDF graphs are existential-conjunctive assertions. 'Rules" is a rather misleading term for a universal implication. All the required logic follows from this. Isnt there something more useful to discuss? Pat -- --------------------------------------------------------------------- IHMC (850)434 8903 home 40 South Alcaniz St. (850)202 4416 office Pensacola (850)202 4440 fax FL 32501 (850)291 0667 cell [email protected] http://www.coginst.uwf.edu/~phayes [email protected] for spam
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