SIG2: Ontology Language Standards

1

• SIG2 Ontology Language Standards
• WebOnt Briefing
• Ian Horrocks
• University of Manchester, UK

2
The Semantic Web Vision

• Web made possible through established standards
• TCP/IP for transporting bits down a wire
• HTTP HTML for transporting and rendering
  hyperlinked text
• Applications able to exploit this common
  infrastructure
• Result is the WWW as we know it
• 1st generation web mostly handwritten HTML pages
• 2nd generation (current) web often machine
  generated/active
• Both intended for direct human processing/interact
  ion
• In next generation web, resources should be more
  accessible to automated processes
• To be achieved via semantic markup
• Metadata annotations that describe
  content/function
• Coincides with Tim Berners-Lee's vision of a
  Semantic Web

3
Ontologies

• Semantic markup must be meaningful to automated
  processes
• Ontologies will play a key role
• Source of precisely defined terms (vocabulary)
• Can be shared across applications (and humans)
• Ontology typically consists of
• Hierarchical description of important concepts in
  domain
• Descriptions of properties of instances of each
  concept
• Degree of formality can be quite variable (NL
  logic)
• Increased formality and regularity facilitates
  machine understanding
• Ontologies can be used, e. g.
• To facilitate buyer seller communication in e-
  commerce
• In semantic based search
• To provide richer service descriptions that can
  be more flexibly interpreted by intelligent agents

4
Historical Perspective

• Ontology Inference Layer OIL
• Developed by group of (largely) European
  researchers many of whom worked on OntoKnowledge
  project
• DAML Ontology Language DAML-ONT
• Developed by group of (largely) US researchers
  working in DAML program
• Efforts merged in DAMLOIL
• Further development by EU/US joint committee
• W3C WebOntology group chartered
• Tasked to develop W3C standard based on DAMLOIL

5
Progress So Far

• Tech Report Working Drafts Released
• Feature synopsis
• High level overview of language features
• User guide and example ontology
• Language reference
• Based on DAMLOIL language specification
• Abstract syntax and semantics
• OIL style macro constructors with MT semantics
• Test cases
• Designed to test implementation conformance

6
Key Differences w.r.t. DAMLOIL

• Semantic layering w.r.t. RDF
• DAMLOIL and RDF semantics not fully compatible
• Problem largely ignored in DAMLOIL
• OWL addresses problem by specifying 3 language
  layers
• Renaming of some constructors
• E.g., hasClass -gt someValueFrom
• No qualified number restrictions
• I.e., cannot describe class of people having 2
  Italian friends
• Modularisation via imports statement
• Semantics based on recursive syntactic closure of
  imported ontologies

7
Semantic Layering Compromise

• Three language layers called (provisionally)
• OWL full
• Union of OWL and RDFS
• OWL DL
• Restricted to DL/FOL fragment (?DAMLOIL)
• OWL Lite
• Subset of OWL DL
• Semantic Layering
• OWL DL semantics OWL full semantics (within DL
  fragment)
• OWL Lite semantics OWL DL semantics (within
  Lite fragment)
• DL semantics are definitive
• If full disagrees with DL (in DL fragment), then
  full is wrong

8
OWL full

• No restriction on use of OWL vocabulary (as long
  as legal RDF)
• Classes as instances (and much more)
• RDF/LBase style model theory
• Still requires axiomatisation
• Reasoning via FOL engines
• But needs (performance killing) axiomatisation
• Semantics should correspond with OWL DL for
  suitably restricted KBs

9
(No Transcript)
10
OWL DL

• Use of OWL vocabulary restricted
• Cant be uses to do nasty things (I.e., modify
  OWL)
• No classes as instances
• Defined by abstract syntax
• Standard DL/FOL model theory (definitive)
• Direct correspondence with (first order) logic
• Reasoning via DL engines
• Some problems with oneOf/inverse
• Reasoning for full language via FOL engines
• No need for axiomatisation (unlike full)
• Would need built in datatypes for performance

11
(No Transcript)
12
OWL Lite

• Like DL, but fewer constructs
• No explicit negation or union
• But can be captured
• Restricted cardinality (zero or one)
• No nominals (oneOf)
• Semantics as per DL
• Some arguments for classes as instances
• But now seem to have evaporated
• Reasoning via standard DL engines (datatypes)
• E.g., Cerebra

13
Next Steps

• Final review of technical reports
• This is where we come in
• Face to face meeting in Manchester, 9-10 January
• Resolve any outstanding issues
• Discuss implementation issues
• Last call in late January
• W3C recommendation (we hope) by spring 2003

14
Key Documents

• Feature synopsis
• User guide and example ontology
• Language reference
• Abstract syntax and semantics
• Test cases