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Layering the Semantic Web: Problems and Directions


by Peter F. Patel-Schneider Dieter Fensel

Abstract

The Resource Description Framework and the Resource Description Framework Schema Specification are supposed to be the foundations of the Semantic Web, in that all other Semantic Web languages are to be layered on top of them. It turns out that such a layering cannot be achieved in a straightforward way. This paper describes the problem with the straightforward layering and lays out several alternative layering possibilities. The benefits and drawbacks of each of these possibilities are presented and analyzed.

Semantic Web Vision

  1. Bring structure to web pages
  2. Permit software agents to carry out sophisticated tasks for users
  3. Extension of the current web

(Tim Berners-Lee, James Hendler, and Ora Lassila. ``The Semantic Web''. Scientific American, May 2001.)

Requirements for Semantic Web Languages

Form: The languages used in the semantic web need well-defined syntax.

Meaning: The languages used in the semantic web need well-defined semantics.

Semantic Web Tower

Semantic Web Tower

Semantic Web Tower (from Tim Berners-Lee)

Elements of the Semantic Web Tower

The Current Vision of the Semantic Web Tower

Rationale for the Current Vision

Problems with the Semantic Web Vision

  1. Disconnects at the Foundation
    • The XML meaning is not used, so data written in XML cannot be used in the Semantic Web.
    • XML Schema is not used in the Semantic Web languages.
  2. An Inadequate Basis
    • RDF is inadequate for providing either syntax or semantics for the entire Semantic Web.

RDF is an Inadequate Basis for the Semantic Web

An Inadequate Basis - Syntax Problems

An Inadequate Basis - Semantic Problems

Semantic Problems - A Theory of Classes

A Desirable Inference:

Premises:
John is an instance of Student.
John is an instance of Employee.
Conclusion:
John is an instance of the intersection of Student and Employee.

Semantic Problems - A Theory of Classes

A Unfortunate Inference:

Premise:
Conclusion 1:
rdf:type is an instance of the restriction whose instances do not have an rdf:type link to the restriction itself.
Conclusion 2:
rdf:type is not an instance of the restriction whose instances do not have an rdf:type link to the restriction itself.

Options for Layering on Top of RDF

  1. Limit inferences in the higher layers
  2. Use a different semantics for the higher layers
  3. Allow for syntactic extensions
  4. Combine the previous two options

Limiting Inference in the Higher Layers

Using a Different Semantics

Extending the Syntax

Breaking the RDF Stranglehold

A Possible Ontology Language

An Ontology Language for the Semantic Web

A New Semantic Web Vision