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/interaction 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 DAML+OIL –Further development by EU/US joint committee W3C WebOntology group chartered –Tasked to develop W3C standard based on DAML+OIL

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 DAML+OIL 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. DAML+OIL Semantic layering w.r.t. RDF –DAML+OIL and RDF semantics not fully compatible –Problem largely ignored in DAML+OIL –OWL addresses problem by specifying 3 language “layers” Renaming of some constructors –E.g., hasClass -> 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 (  DAML+OIL) –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

10. OWL DL Use of OWL vocabulary restricted –Can’t 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

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