Learning OntologiesBy Alexander De Leon

Feb 9, 2009

What is an Ontology?“An ontology is a specification of a conceptualization”

Subject Domain(part of the world)

Person

Male

FemaleFather Mother

John MariaisMarriedTo

Conceptualization (Concepts, Objects, Relationships)

Specification

Ontology

What is an Ontology?Ontologies are also Computational Artifacts (like programs)

Ontology Machine(Reasoner)

∑ ⊨ ...

Inferences or Entailments

INPUT OUTPUT

What can we do with Ontologies?

Ontologies allow us to represent domain knowledge, so that we can:

Share common understanding.

Enable reuse.

Make domain assumptions explicit.

Separate domain knowledge from operational knowledge.

What can we do with Ontologies?

In information systems, manage of information is separated from the application code.

A set of services are require for the application to access the information (e.g. querying)

Ontologies offer a different set of information services than those found on XML and RDBMS.

Information component

Applicationlogic

Services

Reasoning Services in OWL-DL

Consistency checkingSubsumptionSatisfiabilityEntailmentInstance checkingQuery AnsweringOthers: explanations, approximations, etc.

Semantics vs. Syntax

XML Schema (syntactic constraints):<xs:element name= "ParentOfThree" > ... <xs:element ref= "Child" minOccurs= "3" maxOccurs= "3" /> ....</xs:element>

Valid XML Document:<ParentOfThree name= "Aphrodite" > <Child name= "Eros" /> <Child name= "Phobos" /> <Child name= "Himeros" /></ParentOfThree>

Invalid XML Document:<ParentOfThree name= "Aphrodite" > <Child name= "Eros" /></ParentOfThree>

Semantics vs. Syntax

Ontology Concept:ParentOfThree ≡ Person ⊓ ( = 3 hasChild)

Consistent:<ParentOfThree rdf:about="#Aphrodite"> <hasChild rdf:resource="#Eros"/> <ParentOfThree>

Also consistent:<ParentOfThree rdf:about="#Aphrodite"> <hasChild rdf:resource="#Eros"/>

<hasChild rdf:resource="#Phobos"/> <hasChild rdf:resource="#Himeros"/> <hasChild rdf:resource="#Cupid"/>

<ParentOfThree>

“Open World Semantics”Lack of “Unique Name Assumption”

OWL

OWL-DL & OWL-Lite: Correspondence to Description Logics formalisms.

OWL-DL : Maximum expressivity while maintaining computational completeness and decidability.

OWL-Full: OWL vocabulary with syntactic freedom of RDF and no computational guarantees.

OWL-Lite

OWL-DL

RDFS / OWL-Full

Sublanguages of OWL

OWL

OWL-DL & OWL-Lite: Correspondence to Description Logics formalisms.

OWL-DL : Maximum expressivity while maintaining computational completeness and decidability.

OWL-Full: OWL vocabulary with syntactic freedom of RDF and no computational guarantees.

Sublanguages of OWL

OWLEntities of an OWL ontology are identified by URIs (e.g. http://dumontierlab.com/students/alex)

The basic entities are:

Class (a concept, e.g. Person)

Individual (an object, e.g. John)

Object Property (a relationship between two individuals, e.g. loves(John, Susan) )

Data Property (an association between an individual an a piece of data, e.g. age(Alex, 26) )