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.

• Otherwise software agents cannot determine what constructs they are using.

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

• Otherwise software agents cannot determine what the constructs that are using mean.

Semantic Web Tower

Semantic Web Tower (from Tim Berners-Lee)

Elements of the Semantic Web Tower

• XML - eXtensible Markup Language , including namespaces -
  • "XML is the basis for RDF and the Semantic Web" (XML in 10 points)
• XML Schema -
  • XML Schemas "provide a means for defining the structure, content, and semantics of XML documents"
• RDF - Resource Description Framework -
  • RDF is "a foundation for processing metadata"
• RDF Schema -
  • RDF Schema provides mechanisms for defining RDF vocabularies
• DAML+OIL , an ontology language -
  • DAML+OIL uses RDF syntax, but does not use the RDF semantic
• other elements do not yet exist
  • new ontology language (OWL) under development

The Current Vision of the Semantic Web Tower

• XML is the base syntactic language for the Semantic Web.
  • All Semantic Web languages can be written in XML dialects.
  • The meaning of Semantic Web language is not necessarily related to their XML meaning.
• RDF is the base language for the Semantic Web.
  • All Semantic Web languages use RDF for their syntax.
  • All Semantic Web languages build on RDF for their semantics.

Rationale for the Current Vision

• XML is supposed to be the mechanism for defining all Web languages.
  • XML Schema, XML ..., RDF, RDF Schema, OIL, ....
  • Different languages are in different documents.
  • Base data is contained in XML documents.
  • XML systems can parse all Web documents, but will not understand them.
• RDF is supposed to be the mechanism for defining all Semantic Web languages.
  • RDF Schema, ....
  • Different languages are in the same document.
  • All information is contained in RDF documents.
  • RDF systems should be able to parse and (partially) understand all Semantic Web documents.

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

• RDF syntax is triples, with little or no other organization.
  • subject, predicate, object
  • No possibility of variables, etc.
• All RDF triples are asserted facts.
  • No possibility of disjunctions, etc.

An Inadequate Basis - Syntax Problems

• Triples make syntax unnatural.
  • Restrictions have to be split up.
  • Collections have to split up.
    • DAML+OIL defines a special syntax extension for collections.
• Triples allow for deviant syntax.
  • Restrictions with too many pieces.
  • Collections with missing or multiple components.
• Triples allow additions at any time.
  • Can't forbid new triples attached to old syntax.

An Inadequate Basis - Semantic Problems

• Everything is triples; all triples have meaning.
• All aspects of syntax contribute to meaning.
  • e.g., ordering in collections
  • the intersection of Student and Employee is different from the intersection of Employee and Student
• All syntax refers to something.
  • e.g., descriptions live in the domain
  • to infer membership in a description, the description has to exist
  • leads to semantic paradoxes

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.

• In an RDF extension, this requires that the intersection automatically exists whenever Student and Employee exists.
• Many more of these examples can be devised.

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

• Remove some of the desirable inferences
• e.g., John a Student and John an Employee does not imply that John is in an intersection of Student and Employee
• Leads to a difficult-to-explain formalism
• Also difficult to develop the formalism
  • just which inferences are retained

Using a Different Semantics

• Higher levels have their own meaning for triples
• similar to the relationship between XML and RDF
• Means that higher levels don't respect RDF meaning of constructs
  • not true to the vision of the Semantic Web

Extending the Syntax

• Higher levels use their own syntax for their own constructs
  • like the situation between propositional logic and first-order logic
• Extended syntax does not have the limitations of RDF triples
  • can be given whatever meaning is needed in the higher level

Breaking the RDF Stranglehold

• Treat RDF (or XML) as a language for providing facts.
• Still use XML for syntax.
• Use different syntaxes for the other Semantic Web languages.
  • Allow RDF syntax, but also allow extensions.
  • E.g., syntax like OIL's for Ontologies.
  • E.g., more-normal syntax for logic languages.
• Use the RDF model theory as the basis for the model theory of the other Semantic Web Languages.
  • Other Semantic Web languages extend the RDF semantics.

A Possible Ontology Language

• Expressive power similar to DAML+OIL
• Syntax either
  • extension of RDF syntax
  • RDF triples, but considered as syntax, and only those that make sense together
• Use RDF for information about individuals

A New Semantic Web Vision

• Not from me!
• Building more of the Semantic Web may expose more problems.
  • trust, certainty, ...
• However, the more flexible yet integrated the better!
  • want information to flow between the levels
  • a single syntax and a single semantics is just too restrictive