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Layering the Semantic Web: Problems and Directions

Layering the Semantic Web: Problems and Directions. Peter F. Patel – Schneider Dieter Fensel Presenter: Mahek Jasani (USC). CONTENTS. What is in the paper? Semantic Web Layers Possible Ways To L ayer OWL O n Top O f RDF Problem W hen Layering OWL On Top Of RDF(s) Possible Solutions

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Layering the Semantic Web: Problems and Directions

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  1. Layering the Semantic Web: Problems and Directions Peter F. Patel – Schneider Dieter Fensel Presenter: MahekJasani (USC)

  2. CONTENTS • What is in the paper? • Semantic Web Layers • Possible Ways To Layer OWL On Top Of RDF • Problem When Layering OWL On Top Of RDF(s) • Possible Solutions • Conclusions

  3. What Is In The Paper • Brief Description Of Semantic Web Layers • Problems and Possible Solutions to Layer OWL on top of RDF(s)

  4. Layers of Semantic Web

  5. Possible Ways to Layer OWL on top of RDF What is the relationship between various layers in Semantic Web?? • Syntax (Same?? Extension?? Subset?? Different??) • Semantics (Same?? Extension?? Different??) Since this is semantic web, we expect that meaning be retained from one layer to the next. BUT!!!

  6. Possible Ways to Layer OWL on top of RDF There is a point where semantic web moves out of the semantic realm. At this point we do not expect the semantics of the layer, if any, be preserved. e.g. UniCode/URI – not all Unicode strings in RDF are strings. Strings are given different meaning in semantic web e.g. Semantics of XML doc are not retained in RDF. RDF provides its own meaning for XML documents. The only semantic layering is between RDF and RDF(S)

  7. Possible Ways to Layer OWL on top of RDF • Same-Syntax Semantic Extension • Syntax and Semantic Extension • Same Syntax, but Diverging Semantics • Differing Syntax & Semantics

  8. Possible Ways to Layer OWL on top of RDF Which Way do You Think Is The Best ? ? ?

  9. Problem When Layering OWL on top of RDF • The most attractive way would be to use the same layering relationship as that between RDF and RDF Schema. • i.e. OWL would have the same syntax as RDF and the semantics would be an extension of the semantics of RDF(s). • BUT!!! It is not possible!!! • IT LEADS TO SEMANTIC PARADOXES • What’s that???

  10. Semantic Paradox • Analogous to Russell’s Paradox (Set Theory) that destroyed the initial formalization of set theory. • OWL layered on top of RDF Schema as a same-syntax extension has the same problem. • To make the logical foundations of classes in the extension work correctly, there has to be a large collection of built in classes in any model. • But this collection includes the class that is defined as those resources that does not belong to that class.

  11. RDF & RDFS • RDF - <subject property object> • The unusual part is that properties are also URIs!! • RDF provides rdf:type (property) that links a resource to the types that resource belongs to. • RDFS extends this by creating a theory of classes and properties. (URI / Node ID) (URI) (URI / Node ID/ Literal)

  12. More RDFS • Classes in RDFS are those resources that can have members. • It defines several built-in classes, including rdfs:ClassThe class of all classes rdfs:ResourceThe class of all resources and several relationship classes, including rdfs:subClassOfThe subclass relationship BUT!! RDFS theory of classes and properties is weak!! WHY??

  13. Back to the Problem • Is it possible for RDF Schema to provide defined classes – classes that give a formula that determines which resources belong to them??? • OWL intends to provide an even richer theory of classes and properties, allowing for defined classes and more relationships between classes. • Some of the defined classes are called “restrictions” in OWL • It is this richer theory of classes that clashes with the underlying principles of RDF(S), resulting in paradoxes!! How??

  14. Back to the Problem • It’s mainly because restrictions are also resources!!

  15. Back to the Problem Consider for example, RDF(S) Class – Person Property – child Object – John (no outgoing child relationships) • It is possible to create restriction corresponding to those resources whose children all belong to person. • John belongs to this restriction because he has no children and so all John’s children belong to Person. • We need such interpretation to be a model for John belonging to this restriction and that includes John as a member. • But restrictions are resources and this can only happen if there is a resource corresponding to the restriction.

  16. Back to the Problem • Consider the restriction that states it is precisely those resources that have at most zero values for the property owl:type that belong to the class that consists of restriction itself. • This restriction is the restriction that consists of those resources that does not belong to it. • But, it is paradoxical!! • If it belongs to the restriction then it does not, but if it does not, then it does!

  17. Possible Solutions What could be the approach?? • Limiting entailment • Give up on entailment that require presence of problematic restrictions in all interpretations. 2. Syntactic and Semantic extension • Make the restrictions of OWL be new syntax 3. Same syntax but diverging Semantics • Have the same syntax as RDF but does not abide by all of the RDF(S) meanings. 4. Diverging Syntax and Diverging Semantics

  18. CONCLUSION • The biggest problem does not come from any particular aspect of RDF(S) itself but instead from a vision of developers and users of RDF(S) • In their vision RDF encompasses the whole semantic web. • The higher layers of semantic web serve to provide extra meaning for the information that goes beyond the scope of RDF, but they are neither free to extend the syntax of RDF nor they are free to modify the basic meaning provided by the RDF

  19. QUESTIONS??

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