74.419 Artificial Intelligence 2004

Ontology

• Ontology Languages

• Semantic Web

• "Real" Ontologies

Ontology Languages and Ontologies

Semantic Web – Ontology languages for Internet organization and search (DAML, OIL, OWL)

Wordnet – based on linguistic descriptions

Cyc – Knowledge Base organization system

Ontolingua – KB development tool (with predefined Ontology), Stanford U.

Microcosmos – Ontology developed for Computational Linguistics, CRL

Semantic Web - Ontology

General Ontology Languages (similar to LOOM, based on DL)

for the WWW

Semantic WebDefinition: The Semantic Web is the representation of data on the World Wide Web. It is a collaborative effort led by W3C with participation from a large number of researchers and industrial partners. It is based on the Resource Description Framework (RDF), which integrates a variety of applications using XML for syntax and URIs for naming.

"The Semantic Web is an extension of the current web in which information is given well-defined meaning, better enabling computers and people to work in cooperation." -- Tim Berners-Lee, James Hendler, Ora Lassila, The Semantic Web, Scientific American, May 2001

OWL, OIL, and DAML

• DAML - Agent Markup Language

• OIL - Ontology Inference Engine

• DAML+OIL - "semantic markup language for Web resources"

• integrate Ontology Language with the Resource Description Framework (RDF)

• OWL - Web Ontology Language (follower of DAML and OIL; based on DL)

• Developed by the WWW Consortium: W3C.

DAML+OIL

DAML+OIL is a semantic markup language for Web resources. It builds on earlier W3C standards such as RDF and RDF Schema, and extends these languages with richer modelling primitives. DAML+OIL provides modelling primitives commonly found in frame-based languages. DAML+OIL (March 2001) extends DAML+OIL (December 2000) with values from XML Schema datatypes. [...]

The language has a clean and well defined semantics.

DAML+OIL - Example

Concept Definition (multiple superclasses):

a Man is a Male Person

<daml:Class rdf:ID="Man"> <rdfs:subClassOf rdf:resource="#Person"/> <rdfs:subClassOf rdf:resource="#Male"/> </daml:Class>

DAML+OIL - Example

Relation Definition

hasParent is a relation between animals

<daml:ObjectProperty rdf:ID="hasParent">

<rdfs:domain rdf:resource="#Animal"/>

<rdfs:range rdf:resource="#Animal"/>

</daml:ObjectProperty>

OWL - Main Language Constructs

RDF Schema Features: • Class • rdf:Property • rdfs:subClassOf • rdfs:subPropertyOf • rdfs:domain • rdfs:range • Individual

OWL - Equality Expressions

(In)Equality:

• equivalentClass

• equivalentProperty

• sameAs (for Individuals)

• differentFrom (for Individuals)

• allDifferent (for several Individuals)

OWL - Combine Class Expressions

• Boolean Combinations of Class Expressions:

• unionOf

• intersectionOf

• complementOf

OWL - Class Axioms

Class Axioms:

• oneOf (describe enumerated classes)

• disjointWith

• equivalentClass (for class expressions)

• rdfs:subClassOf (for class expressions)

OWL- Properties

Property Characteristics:

• InverseOf

• TransitiveProperty

• SymmetricProperty

• FunctionalProperty

• InverseFunctionalProperty

OWL- Properties

Property Type Restrictions:

• allValuesFrom

• someValuesFrom

Filler Information:

• hasValue

Arbitrary Cardinality:

• minCardinality

• maxCardinality

• cardinality

Reasoning with Ontologies

• Classifier

• Consistency Check

• Check for Implied Relatonships

• Compare and Combine Different Ontologies

Ontology - Contents

How to fill this framework given by general Ontology Languages?

What is Ontology?

The analysis and description of how the world is structured. (my definition)

"That department of the science of metaphysics which investigates and explains the nature and essential properties and relations of all beings, as such, or the principles and causes of being." (Webster's)

"the metaphysical study of the nature of being and existence" (WordNet)

"<philosophy> A systematic account of Existence." (Free On-line Dictionary of Computing)

What is Ontology?

"<artificial intelligence> (From philosophy) An explicit formal specification of how to represent the objects, concepts and other entities that are assumed to exist in some area of interest and the relationships that hold among them." (Free On-line Dictionary of Computing)

"Real" Ontology

Ontology• looks for semantic and ontological primitives

and concepts to describe aspects or parts of “the world”

• aim of research is to develop KBs which can be shared and commonly used in various contexts, i.e. for different applications and different fields (e.g. for NLP, reasoning, planning etc.)

• relates to psychology and philosophy, as well as data bases and object-oriented programming systems

Ontologies – Background

Ontology – Conceptual Hierarchy

Wittgenstein (philosophy and language) – “Family Resemblances” instead of exact concept definitions

Eleanor Rosch (cognitive psychology) – Prototype Theory; natural concepts

Constructing Ontologies

Basic elements of ontology languages• Concepts & IS-A Hierarchy• Roles, Relations w. specification• Features, Attributes, Slots w. specification

-------------------------------• Instances (concrete objects)• Assertions (facts about objects in the world)

-------------------------------• Axioms (define the world; describe

general rules of the world)

Basic Concepts in Ontologies

• Physical Objects– Stuff and Things; Composite Objects– Fluids– Substances

• Actions and Events– happen in time; agent or natural cause or no cause– modeling dynamic aspects problematic

• Abstract Objects– abstract concepts (ideas) like 'freedom'

– difficult to define and describe

Basic Concepts in Ontologies

• Physical structures – Time– Space

• Mathematical structures– Sets, Groups, ...

Web References

Description Logics Page www.dl.kr.org

LOOM www.isi.edu/isd/LOOM/LOOM-HOME.html

Ontology Page www.kr.org.top

Ontolingua Home Page www.ksl.stanford.edu/software/ontolingua/

Open Cyc www.opencyc.org/