1 Sean Bechhofer Information Management Group University of Manchester, UK Reasoning: Who Gives a Hoot?

2 The Semantic Web Most existing Web resources only human understandable –Markup (HTML) provides rendering information –Textual/graphical information for human consumption Semantic Web aims at machine understandability –Semantic markup will be added to web resources –Markup will use Ontologies for shared understanding –Requirement for an ontology language

3 Some History OIL –RDFS based syntax –Based on frame-based language –Strong emphasis on formal rigour –Semantics in terms of Description Logic language DAML-ONT –Developed by DAML Programme –Extended RDFS with constructors from OO and frame-based languages –Rather weak semantic specification Problems with machine interpretation Problems with human interpretation

4 Some More History DAML+OIL –Merging of DAML-ONT and OIL –Joint EU/US committee –Basically a DL with an RDFS-based syntax –Submitted to W3C as basis for standardisation

5 OWL Based on DAML+OIL –Some slight modifications, e.g. removal of qualified cardinality constraints Layers of expressiveness: –OWL Lite Restricted cardinality expressions –OWL Fast/DL (~DAML+OIL) Enumerations Boolean Expressions Arbitrary Expressions in Axioms –OWL Large Loads of other stuff…

6 DAML+OIL/OWL Languages for defining (class-based) ontologies –Classes Collections of domain objects Constructors for describing and defining classes –Boolean expressions –Explicit quantification of restrictions –Properties (Binary) relationships between domain objects Separation of object properties and datatype properties –Individuals Specific named objects in the domain

7 Migratory Paths You don’t have to use all the expressiveness Simple taxonomies can be defined Further refined and elaborated at a later date Use reasoning to assist in this elaboration –maintaining the hierarchy and internal consistency of the ontology Supports incremental development

8 Semantics What does an expression in an ontology mean? The semantics of DAML+OIL tell us precisely how to interpret a complex (class) expression Model theoretic semantics. An interpretation consists of –A domain of discourse (collection of objects). –Functions mapping classes to sets of objects properties to sets of pairs of objects –Rules describe how to interpret the constructors and tell us when an interpretation is a model Well defined semantics are vital if we are to support machine interpretability

9 Reasoning Due to the presence of the semantics, we can reason about OWL & DAML+OIL models –Subsumption reasoning: tells us when all instances of one class are necessarily instances of another class –Satisfiability reasoning: tells us when it is logically impossible to have instances of a class Set of operators/axioms restricted so that reasoning is decidable Restricted expressiveness facilitates provision of reasoning services –Known algorithms –Implemented systems –Evidence of empirical tractability

10 Why Reasoning Services? Reasoning is important for: Ontology design –Check class consistency and (unexpected) implied relationships –Particularly important with large ontologies/multiple authors Ontology integration –Assert inter-ontology relationships –Reasoner computes integrated class hierarchy/consistency Ontology deployment –Determine if set of facts are consistent w. r. t. ontology –Determine if individuals are instances of ontology classes The Semantic Web needs a logic on top Henry Thompson

11 OilEd Designed initially to demonstrate the expressive power of OIL & DAML+OIL. A light-weight ontology editor. Not a knowledge base construction tool. A platform to explore how to use a reasoner.

12 OilEd Philosophy Tabbed panes for classes, slots, individuals and axioms. Basic component is the frame description –Specifies superclasses and slot constraints. –Controls for addition, removal and editing of superclasses and constraints –Provides a “user-friendly” face on top of DAML+OIL. Replacing the lost frame nature. Different editors for different expression types Simple hierarchical views. –No frills.

13 Interesting Features Arbitrary class expressions can be used as slot fillers. –Extending the frame-based paradigm. –Boolean connectives. Primitive & defined classes. A number of slot constraint types. –Explicit quantification. Slot hierarchies. Axioms –Disjointness, covering and equality. Possibility of using a reasoner.

14 Reasoning Spots inconsistent definitions –e.g. contradictions in cardinality constraints or value restrictions Organises the classification hierarchy –Discovering new superclasses –Particularly useful when using defined classes Subtle side-effects –Superclasses inferred due to domain/range restrictions –Information scattered throughout the model can affect classification

15 Reasoning Reasoning via a standard DL API –Based on satisfiability and subsumption reasoning –Using the new DIG protocol Allows the use of other reasoners, e.g. RACER or Cerebra One shot connection to the reasoner –Translate ontology to equivalent DL axioms –Send to reasoner and then make requests –Allows temporary inconsistency What do you do with the results? –Reorganize hierarchy or just add things in? –What if you’re connected all the time?

16 Demo!

17 Hard Things Hierarchical displays –Difficult with multiple inheritance and big hierarchies DAML+OIL/OWL shift from frames –Original OIL language was much more frame-like than DAML+OIL or OWL –Can be hard to preserve the original intention of the modeller Expression (and axiom) editing is clunky –Expression migration can be difficult –But it’s fundamentally quite a hard task –Lack of a “human readable” syntax

18 More Hard Things! Concrete Types aren’t very well supported –XML Schema types –Little reasoning support (not OilEd’s fault!) Not much support for instances –Reasoning over enumerated classes is a (reasonable) hack –But remember: OilEd was not intended as a full-scale knowledge base construction tool Explanation –Why did that happen?

19 Good Things Lots of interest –>1600 registered downloads of the latest version. A demonstration of the reasoner & the application of reasoning Useful in understanding how one might use a reasoner within such an application –A focus for discussion

20 Users Projects –GONG, myGrid, AstroGrid Industry –Boeing, HP Teaching –University of Manchester, Free University of Amsterdam, Prague University of Economics, UPM (Madrid), TZI (Bremen),… Others –EML, Hamburg, Maryland, Erlangen-Nürnberg, China, Turkey, Kathmandu(!),…

21 Users

22 Further Work Rebuild it properly –Better modular structure –Facilitating community involvement and third-party extensions Integration with existing tools. –KAON –Protégé? Improve Concrete Datatype Support. –Likely based on XML Schema (following OWL) Better Ontology Engineering –Version & Change Management Instance Store/KB Methodology support.

23 Summary Building and maintaining ontologies is hard –Need support from intelligent tools Frame based ontology editors are one such tool –Familiar to and liked by many users –Intuitive (for some) interface –Facilitate gradual refinement of ontology design Frame based editors can be extended with –power and semantic clarity of expressive DL –reasoning support But…it won’t do everything!

24 Acknowledgements: The original development of OilEd was supported by the Free University of Amsterdam, Interprice GmbH (now Semantic Edge) and the University of Manchester OilEd owes obvious debts to existing ontology editors such as Protégé.