Short definition of an ontology

1. Learning Ontologies
By Alexander De Leon
Feb 9, 2009

2. What is an Ontology?
“An ontology is a specification of a conceptualization”
Person
Fe
le
m
Ma
al
e
Father Mother
John Maria
isMarriedTo
Subject Domain Conceptualization
Specification
(part of the world) (Concepts, Objects, Relationships)
Ontology

3. What is an Ontology?
Ontologies are also Computational Artifacts (like
programs)
∑ ⊨ ...
Ontology Inferences or
Machine
Entailments
(Reasoner)
INPUT OUTPUT

4. 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.

5. 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 Application
Services
component logic

6. Reasoning Services in
OWL-DL
Consistency checking
Subsumption
Satisfiability
Entailment
Instance checking
Query Answering
Others: explanations, approximations, etc.

7. Semantics vs. Syntax
XML Schema (syntactic constraints):
<xs:element name= "ParentOfThree" >
...
<xs:element ref= "Child" minOccurs= "3" maxOccurs= "3" />
....
</xs:element>
Valid XML Document: Invalid XML Document:
<ParentOfThree name= "Aphrodite" > <ParentOfThree name= "Aphrodite" >
<Child name= "Eros" /> <Child name= "Eros" />
<Child name= "Phobos" /> </ParentOfThree>
<Child name= "Himeros" />
</ParentOfThree>

8. Semantics vs. Syntax
Ontology Concept:
ParentOfThree ≡ Person ⊓ ( = 3 hasChild)
Consistent: Also consistent:
<ParentOfThree rdf:about="#Aphrodite"> <ParentOfThree rdf:about="#Aphrodite">
<hasChild rdf:resource="#Eros"/> <hasChild rdf:resource="#Eros"/>
<ParentOfThree> <hasChild rdf:resource="#Phobos"/>
<hasChild rdf:resource="#Himeros"/>
<hasChild rdf:resource="#Cupid"/>
<ParentOfThree>
“Open World Semantics”
Lack of “Unique Name Assumption”

9. OWL
Sublanguages of OWL
OWL-Full: OWL vocabulary
with syntactic freedom of RDF
and no computational
guarantees.
OWL-DL & OWL-Lite:
OWL-Lite Correspondence to Description
Logics formalisms.
OWL-DL OWL-DL : Maximum
expressivity while
maintaining computational
RDFS / OWL-Full completeness and
decidability.

10. OWL
Sublanguages of OWL
OWL-Full: OWL vocabulary
with syntactic freedom of RDF
and no computational
guarantees.
OWL-DL & OWL-Lite:
Correspondence to Description
Logics formalisms.
OWL-DL : Maximum
expressivity while
maintaining computational
completeness and
decidability.

11. OWL
Entities 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) )