Acknowledgements

• I'm fortunate to work with and learn from some of the proprietors of the Semantic Web
  • Most notably Tim Berners-Lee and Dan Connolly
• They're not in any way responsible for the opinions presented here
• My colleague Michael Sperberg-McQueen first enumerated for me RDF's particular qualities
  • and got me started on thinking about what significance this might have

A problem for building LKR

• Choosing a knowledge representation
• 'Traditional' languages
  • First-order logic
  • AI-based KR languages
• The new option
  • The Semantic Web

The Semantic Web

• A collection of technologies from the World Wide Web Consortium (W3C)
  • Inspired by Tim Berners-Lee
• The goal of the Semantic Web
  • Reproduce for machines what the original Web has done for people
• Alongside human beings finding information of the Web
• Computers finding information on the Semantic Web
• Distributed development of information resources which can be
  • Discovered
  • Merged
  • Exploited
• All without human participation.

Semantic Web technologies

• Reconstructing basic elements of traditional knowledge representation systems
• To make them 'web-friendly'
  • RDF
    • syntactic
    • low-level
    • two-place relational graph data model
    • for recording assertions
  • OWL
    • semantic
    • higher-level
    • not a new language
    • a collection of classes and relations
    • for use in describing ontologies
• To date, less work on inference or reasoning
  • Connections with description-logic-based inference engines
  • Recently work has started on a rule language

What's not special about the Semantic Web

• Two-place relations
  • understood as forming a graph
• Are nothing special
• An XML serialisation is nothing special
• Designing the language first
  • Then playing 'catch-up' with respect to inference
• Isn't new either, alas

What is special about the Semantic Web

• It's KR for the Web
• That means one thing in particular
  • Coming from the fundamental architecture of the Web
• Everything has a universal name
• URIs (Uniform Resource Identifiers) are used for naming
  • Individuals
  • Classes
  • And relations.
• Usually http URIs
  • http://www.w3.org/2002/07/owl#Class

RDF example

RDF graph

relations

creator(homepage,ora)
name(ora,"Ora Lassila")
email(ora,"Lassila@w3.org")
ako(ora,Person)

RDF triples

http://www.w3.org/Home/Lassila
 http://purl.org/dc/elements/1.1/#creator
http://www.w3.org/staffId/85740
----------
http://www.w3.org/staffId/85740
 ???#Name
"Ora Lassila"
----------
http://www.w3.org/staffId/85740
 ???#Email
"lassila@w3.org"
----------
http://www.w3.org/staffId/85740
 http://www.w3.org/1999/02/22-rdf-syntax-ns#type
http://www.example.org/UpperOntology#Person

The impact of URIs

• RDF graphs are composed of subject-relation-object triples
• All three parts are named with URIs.
• In principle, you can directly merge two independently-constructed RDF-encoded knowledge bases
  • Graph nodes named with the same URI can simply be merged
  • URI comparison is trivial and well-defined

Merging ontologies

• The relations in a factual knowledge base
• Are the nodes (subjects or objects) in an ontology
• So in principle easy to imagine merging multiple ontologies

Practical problems

• Independently developed knowledge bases
  • or ontologies
• Are unlikely to have chosen the same URIs for the same individuals
  • or concepts
• Even when the same URIs do turn up in two graphs
  • You can't just assume identity of real-world denotation or ontological functionality
• This is sometimes called the SW 'Identity Problem'

One clear benefit of URIs

• Using URIs for everything enables the 'follow your nose' strategy
• URIs identify resources
• URIs can be dereferenced
  • At least, http URIs can
• Dereferencing an SW URI should produce useful information about the thing/concept it is being used to identify.

'Follow your nose' example

• Suppose I (or a program) find the following triple in a graph:

  http://www.example.org/Books/MobyDick
    http://purl.org/dc/elements/1.1/#creator
  http://www.example.org/Authors/HermanMelville

• Dereferencing the creator URI yields both human- and machine-readable information about the relation it identifies
  • Its official definition, in natural language/HTML
  • Its place in an RDF-encoded ontology (i.e. an OWL schema)

The key prediction

• The ability to 'follow your nose' will solve the Identity problem.
• Creators of new knowledge resources will find existing related knowledge resources
  • (How?)
• Dereference the URIs of the subjects and relations therein
• Use the ontology they find instead of creating a new one
  • Or at least connect their ontology to the existing one(s)
• Then detecting and exploiting co-reference gets easier

Conclusions, part 1

• The history of knowledge representation sends a clear message
  • Good data models are necessary, but not sufficient, for success
  • The ability to compute with the represented knowledge is the real test.
• In other words, ignore the inference engine problem at your peril.
• It seems likely the Semantic Web project will succeed in one way
  • As a knowledge representation system in its own right
• Much less clear if its larger goal can be achieved
  • The emergence of a virtual world-wide knowledge base.

Conclusions, part 2

• One thing is clearly new and important
  • The Semantic Web project's focus on extending the boundaries of knowledge bases beyond the site at which they are developed
• Sends a clear message to the KR community
• Any knowledge resource creation effort
  • Particularly any one aimed at a large-scale and cooperatively developed knowledge resource
  should look very carefully at the ideology and the technology of the Semantic Web