1 sean bechhofer information management group university of manchester, uk reasoning: who gives a...
TRANSCRIPT
1
Sean BechhoferInformation Management Group
University of Manchester, UK
http://img.cs.man.ac.ukhttp://oiled.man.ac.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?
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(!),…
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!