Introduction toIntroduction toSemantic Web Semantic Web

Ontology LanguagesOntology Languages

Enrico FranconiEnrico Franconi(Grigoris Antoniou)(Grigoris Antoniou)

On this lectureOn this lecture

• Lecturer: Enrico FranconiLecturer: Enrico Franconi– www.inf.unibz.it/~franconi

• Missing lecturer: Grigoris AntoniouMissing lecturer: Grigoris Antoniou– antoniou@icsforth.gr

• Main resourcesMain resources– the chapter in the book– the Introduction to Description Logics web site

• www.inf.unibz.it/~franconi/dl/course/

• Variations:Variations:– Less emphasis on standards, more on semantic meaning

Lecture outlineLecture outline

• What is an ontologyWhat is an ontology• Formalising ontologiesFormalising ontologies• Using ontologiesUsing ontologies

– Accessing data

• Web Ontology LanguagesWeb Ontology Languages– RDF(S)

– OWL

Knowledge Representation:Knowledge Representation:Relating Theory to the World via Formal ModelsRelating Theory to the World via Formal Models

Source: John F. Sowa, Knowledge Representation: Logical, Philosophical, and Computational Foundations

““All models are wrong, but some models are useful!”All models are wrong, but some models are useful!”

What is an ontology (and What is an ontology (and what is it good for)? what is it good for)?

And the answer is ...And the answer is ...

Glossary (wordreference.com)Glossary (wordreference.com)

• ontologyontology  nounnoun11  (  (PhilosophyPhilosophy)  the branch of metaphysics that deals with the nature of being)  the branch of metaphysics that deals with the nature of being22  (  (LogicLogic)  the set of entities presupposed by a theory)  the set of entities presupposed by a theory

• taxonomytaxonomy  nounnoun11    aa  the branch of biology concerned with the classification of organisms into groups based on   the branch of biology concerned with the classification of organisms into groups based on similarities of structure, origin, etc.similarities of structure, origin, etc.bb  the practice of arranging organisms in this way  the practice of arranging organisms in this way22  the science or practice of classification [ETYMOLOGY: 19th Century: from French   the science or practice of classification [ETYMOLOGY: 19th Century: from French taxonomie,taxonomie, from from

Greek Greek taxistaxis order + -nomy] order + -nomy]

• thesaurusthesaurus  nounnoun(plural:  (plural:  -ruses-ruses, , -ri-ri [ [-raı-raı])])11  a book containing systematized lists of synonyms and related words  a book containing systematized lists of synonyms and related words22  a dictionary of selected words or topics  a dictionary of selected words or topics33  (  (rarerare) )

a treasury[ETYMOLOGY: 18th Century: from Latin, Greek: treasure]a treasury[ETYMOLOGY: 18th Century: from Latin, Greek: treasure]

Glossary (wordreference.com)Glossary (wordreference.com)• conceptconcept  nounnoun

11  an idea, esp. an abstract idea  an idea, esp. an abstract ideaexample: example: the concepts of biologythe concepts of biology22  (  (PhilosophyPhilosophy)  a general idea or notion that corresponds to some class of entities and that consists of )  a general idea or notion that corresponds to some class of entities and that consists of the characteristic or essential features of the classthe characteristic or essential features of the class33  (  (PhilosophyPhilosophy) ) aa  the conjunction of all the characteristic features of something   the conjunction of all the characteristic features of something bb  a theoretical construct   a theoretical construct within some theory within some theory cc  a directly intuited object of thought   a directly intuited object of thought d d the meaning of a predicate the meaning of a predicate44  [  [modifiermodifier]  (of a product, esp. a car) created as an exercise to demonstrate the technical skills and ]  (of a product, esp. a car) created as an exercise to demonstrate the technical skills and imagination of the designers, and not intended for mass production or sale[ETYMOLOGY: 16th Century: imagination of the designers, and not intended for mass production or sale[ETYMOLOGY: 16th Century: from Latin from Latin conceptumconceptum something received or conceived, from something received or conceived, from concipereconcipere to take in, conceive to take in, conceive] ]

• contingent contingent adjectiveadjective11  [  [when postpositive, often foll by on or uponwhen postpositive, often foll by on or upon]  dependent on events, conditions, etc., not yet known; ]  dependent on events, conditions, etc., not yet known; conditionalconditional22  (  (LogicLogic)  (of a proposition) true under certain conditions, false under others; not necessary)  (of a proposition) true under certain conditions, false under others; not necessary33  (in systemic grammar) denoting contingency (sense 4)  (in systemic grammar) denoting contingency (sense 4)44  (  (MetaphysicsMetaphysics)  (of some being) existing only as a matter of fact; not necessarily existing)  (of some being) existing only as a matter of fact; not necessarily existing55  happening by chance or without known cause; accidental  happening by chance or without known cause; accidental66  that may or may not happen; uncertain  that may or may not happen; uncertain

• glossary glossary noun noun (plural:  -ries); an alphabetical list of terms peculiar to a field of knowledge with definitions (plural:  -ries); an alphabetical list of terms peculiar to a field of knowledge with definitions or explanations. Sometimes called: glossor explanations. Sometimes called: gloss[ETYMOLOGY: 14th Century: from Late Latin [ETYMOLOGY: 14th Century: from Late Latin glossarium;glossarium; see gloss see gloss22]]

11stst Attempt: Ontologies in CS Attempt: Ontologies in CS

• An ontology is ...An ontology is ...– an explicit specification of a conceptualization [Gruber93]

– a shared understanding of some domain of interest [Uschold, Gruninger96]

• Some aspects and parameters:Some aspects and parameters:– a formal specification (reasoning and “execution”)– ... of a conceptualization of a domain (community)– ... of some part of world that is of interest (application)

• Provides:Provides:– A common vocabulary of terms– Some specification of the meaning of the terms (semantics)– A shared “understanding” for people and machines

Ontology as a philosophical disciplineOntology as a philosophical discipline

• Ontology as a Ontology as a philosophical disciplinephilosophical discipline, which deals with , which deals with the nature and the organization of reality:the nature and the organization of reality:– Ontology as such is usually contrasted with Epistemology,

which deals with the nature and sources of our knowledge [a.k.a. Theory of Knowledge]. Aristotle defined Ontology as the science of being as such: unlike the special sciences, each of which investigates a class of beings and their determinations, Ontology regards all the species of being qua being and the attributes which belong to it qua being" (Aristotle, Metaphysics, IV, 1).

• In this sense Ontology tries to answer to the question: In this sense Ontology tries to answer to the question: What is being? What exists? What is being? What exists? (the nature of being, not an (the nature of being, not an enumeration of “stuff” around us…) enumeration of “stuff” around us…)

Some different uses of the word “Ontology” Some different uses of the word “Ontology” [Guarino’95][Guarino’95]

1. Ontology as a philosophical discipline1. Ontology as a philosophical discipline2. Ontology as a an informal conceptual system2. Ontology as a an informal conceptual system3. Ontology as a formal semantic account3. Ontology as a formal semantic account4. Ontology as a specification of a “conceptualization”4. Ontology as a specification of a “conceptualization”5. Ontology as a representation of a conceptual system5. Ontology as a representation of a conceptual systemvia a logical theoryvia a logical theory

5.1 characterized by specific formal properties5.1 characterized by specific formal properties5.2 characterized only by its specific purposes5.2 characterized only by its specific purposes

6. Ontology as the vocabulary used by a logical theory6. Ontology as the vocabulary used by a logical theory7. Ontology as a (meta-level) specification of a logical 7. Ontology as a (meta-level) specification of a logical

theorytheoryhttp://ontology.ip.rm.cnr.it/Papers/KBKS95.pdf

Ontologies vs ConceptualizationsOntologies vs Conceptualizations

• Given a logical language L ...Given a logical language L ...– ... a conceptualization is a set of models of L which describes

the admittable (intended) interpretations of its non-logical symbols (the vocabulary)

– ... an ontology is a (possibly incomplete) axiomatization of a conceptualization.

conceptualization conceptualization C(L)C(L)

ontologyontology

set of all models M(L)set of all models M(L)logiclogictheoriestheories(consistent sets of (consistent sets of sentences; closed undersentences; closed underlogical consequence)logical consequence)

[Guarino96]http://www-ksl.stanford.edu/KR96/Guarino-What/P003.html

Ontologies vs Knowledge BasesOntologies vs Knowledge Bases

• An ontology is a An ontology is a particularparticular KB, describing facts assumed KB, describing facts assumed to be to be always truealways true by a community of users: by a community of users:– in virtue of the agreed-upon meaning of the vocabulary used

(analytical knowledge):• black => not white

– ... whose truth does not descend from the meaning of the vocabulary used (non-analytical, common knowledge)

• Rome is the capital of Italy

• An An arbitraryarbitrary KB may describe facts which are KB may describe facts which are contingently truecontingently true, and relevant to a particular , and relevant to a particular epistemic epistemic state:state:– Mr Smith’s pathology is either cirrhosis or diabetes

Formal Ontology [Guarino’96]Formal Ontology [Guarino’96]

• Theory of formal distinctionsTheory of formal distinctions– among things

– among relations

• Basic toolsBasic tools– Theory of parthood

• What counts as a part of a given entity? What properties does the part relation have? Are the different kinds of parts?

– Theory of integrity• What counts as a whole? In which sense are its parts connected?

– Theory of identity• How can an entity change while keeping its identity? What are its essential

properties? Under which conditions does an entity loose its identity? Does a change of “point of view” change the identity conditions?

– Theory of dependence• Can a given entity exist alone, or does it depend on other entities?

Ontology: Definition and Scope [Sowa]Ontology: Definition and Scope [Sowa]

• The subject of The subject of ontologyontology is the study of the is the study of the categoriescategories of things that of things that exist or may exist in some domain. The product of such a study, exist or may exist in some domain. The product of such a study, called called an ontologyan ontology, is a catalog of the types of things that are , is a catalog of the types of things that are assumed to exist in a domain of interest assumed to exist in a domain of interest DD from the perspective of a from the perspective of a person who uses a language person who uses a language LL for the purpose of talking about for the purpose of talking about DD. . The types in the ontology represent the The types in the ontology represent the predicatespredicates, , word sensesword senses, or , or concept and relation typesconcept and relation types of the language of the language LL when used to discuss when used to discuss topics in the domain topics in the domain DD. An uninterpreted logic, such as predicate . An uninterpreted logic, such as predicate calculus, conceptual graphs, or KIF, is calculus, conceptual graphs, or KIF, is ontologically neutralontologically neutral. It . It imposes no constraints on the subject matter or the way the subject imposes no constraints on the subject matter or the way the subject may be characterized. By itself, logic says nothing about anything, may be characterized. By itself, logic says nothing about anything, but but the combination of logic with an ontology provides a language the combination of logic with an ontology provides a language that can express relationships about the entities in the domain of that can express relationships about the entities in the domain of interest.interest.

http://users.bestweb.net/~sowa/ontology/index.htm

Ontology: Definition and Scope [Sowa]Ontology: Definition and Scope [Sowa]

• An An informal ontologyinformal ontology may be specified by a catalog of types that may be specified by a catalog of types that are either undefined or defined only by statements in a natural are either undefined or defined only by statements in a natural language. A language. A formal ontologyformal ontology is specified by a collection of names is specified by a collection of names for concept and relation types organized in a partial ordering by the for concept and relation types organized in a partial ordering by the type-subtype relation. Formal ontologies are further distinguished type-subtype relation. Formal ontologies are further distinguished by the way the subtypes are distinguished from their supertypes: an by the way the subtypes are distinguished from their supertypes: an axiomatized ontologyaxiomatized ontology distinguishes subtypes by axioms and distinguishes subtypes by axioms and definitions stated in a formal language, such as logic or some definitions stated in a formal language, such as logic or some computer-oriented notation that can be translated to logic; a computer-oriented notation that can be translated to logic; a prototype-based ontologyprototype-based ontology distinguishes subtypes by a comparison distinguishes subtypes by a comparison with a typical member or with a typical member or prototypeprototype for each subtype. Large for each subtype. Large ontologies often use a mixture of definitional methods: formal ontologies often use a mixture of definitional methods: formal axioms and definitions are used for the terms in mathematics, axioms and definitions are used for the terms in mathematics, physics, and engineering; and prototypes are used for plants, physics, and engineering; and prototypes are used for plants, animals, and common household items. animals, and common household items. ..

http://users.bestweb.net/~sowa/ontology/index.htm

Why develop an ontology?Why develop an ontology?

• To make domain assumptions explicitTo make domain assumptions explicit– Easier to change domain assumptions

– Easier to understand, update, and integrate legacy data

data integration

• To separate domain knowledge from operational To separate domain knowledge from operational knowledgeknowledge– Re-use domain and operational knowledge separately

• A community reference for applicationsA community reference for applications• To share a consistent understanding of what information To share a consistent understanding of what information

means.means.

[Carole Goble, Nigel Shadbolt, Ontologies and the Grid Tutorial][Carole Goble, Nigel Shadbolt, Ontologies and the Grid Tutorial]

What is being shared? What is being shared?

MetadataMetadata• Data describing the content and meaning of resources and services.Data describing the content and meaning of resources and services.

• But everyone must speak the same language…But everyone must speak the same language…

TerminologiesTerminologies• Shared and common vocabulariesShared and common vocabularies

• For search engines, agents, curators, authors and users For search engines, agents, curators, authors and users

• But everyone must mean the same thing…But everyone must mean the same thing…

OntologiesOntologies• Shared and common understanding of a domainShared and common understanding of a domain

• Essential for search, exchange and discoveryEssential for search, exchange and discovery

Ontologies aim at sharing meaningOntologies aim at sharing meaning

[Carole Goble, Nigel Shadbolt, Ontologies and the Grid Tutorial][Carole Goble, Nigel Shadbolt, Ontologies and the Grid Tutorial]

Origin and HistoryOrigin and History

• Humans require words (or at least symbols) to communicate efficiently. The mapping of words to things is indirect. We do it by creating concepts that refer to things.

• The relation between symbols and things has been described in the form of the meaning triangle:

“Jaguar“

Concept

Ogden, C. K. & Richards, I. A. 1923. "The Meaning of Meaning." 8th Ed. New York, Harcourt, Brace & World, Inc

before: Frege, Peirce; see [Sowa 2000]

[Carole Goble, Nigel Shadbolt, Ontologies and the Grid Tutorial][Carole Goble, Nigel Shadbolt, Ontologies and the Grid Tutorial]

Human and machine communicationHuman and machine communication

• ... MachineAgent 1

Things

HumanAgent 2

Ontology Description

MachineAgent 2

exchange symbol,e.g. via nat. language

‘‘JAGUAR“

Internalmodels

Concept

Formalmodels

exchange symbol,e.g. via protocols

MA1HA1 HA2

MA2

Symbol

commit commit

a specific domain, e.g.animals

commitcommitOntology

Formal Semantics

HumanAgent 1

MeaningTriangle

[Maedche et al., 2002]

From Philosophy (again)From Philosophy (again)

• Aristotle (some time ago…):Aristotle (some time ago…):– Concepts

• Genus

• Differentiae

• Cognitive ScienceCognitive Science– Inter-related Frames

– Features

An explicit description of a domainAn explicit description of a domain

• ConceptsConcepts (class, set, type, predicate)(class, set, type, predicate) – event, gene, gammaBurst, atrium,

molecule, cat• PropertiesProperties of concepts and of concepts and

relationshipsrelationships between them between them (slot)(slot)

– Taxonomy: generalisation ordering among concepts isA, partOf, subProcess

– Relationship, Role or Attribute: functionOf, hasActivity location, eats, size

animal

rodent cowcat

mouse

eats

dog

domesticvermin

[Carole Goble, Nigel Shadbolt, Ontologies and the Grid Tutorial][Carole Goble, Nigel Shadbolt, Ontologies and the Grid Tutorial]

ConceptsConcepts

• Primitive concepts: Primitive concepts: – properties are necessary

– Globular protein must have hydrophobic core (but a protein with a hydrophobic core need not be a globular protein)

• GlobularProtein ⊑ has-a.HydrophobicCore

• Defined concepts: Defined concepts: – properties are necessary + sufficient

– Eukaryotic cells must have a nucleus. • EukaryoticCell has-a.Nucleus

– Every cell that contains a nucleus must be Eukaryotic.

[Robert Stevens][Robert Stevens]

What is a concept?What is a concept?

Different communities have different notions on what a concept Different communities have different notions on what a concept means:means:

– Formal concept analysis (see http://www.math.tu-dresden.de/~ganter/fba.html) talk about formal concepts

– Description Logics (see http://dl.kr.org/): They talk about concept labels

– ISO-704:2000 – Terminology Work: (see http://www.iso.ch/)

– Often the classical notion of a frame in AI or a class in OO modeling is seen as equivalent to a concept.

Formal Concept Analysis (FCA)Formal Concept Analysis (FCA)

Concept Lattice

Formal Concept AnalysisFormal Concept Analysis

[Sowa, http://users.bestweb.net/~sowa/misc/mathw.htm]

An explicit description of a domainAn explicit description of a domain

• ConstraintsConstraints or or axiomsaxioms on properties and concepts: on properties and concepts: – value: integer – domain: cat– cardinality: at most 1– range: 0 <= X <= 100– oligonucleiotides < 20 base pairs– cows are larger than dogs– cats cannot eat only vegetation– cats and dogs are disjoint

• ValuesValues or or concrete domainsconcrete domains– integer, strings– 20, trypotoplan-synthetase

animal

rodent cowcat

mouse

eats

dog

domesticvermin

[Carole Goble, Nigel Shadbolt, Ontologies and the Grid Tutorial][Carole Goble, Nigel Shadbolt, Ontologies and the Grid Tutorial]

An explicit description of a domainAn explicit description of a domain

• IndividualsIndividuals or or InstancesInstances – sulphur, trpA Gene, felix

• NominalsNominals– Concepts that cannot have instances

– Instances that are used in conceptual definitions

– ItalianDog = Dog bornIn Italy

• InstancesInstances– An ontology =

concepts+properties+axioms+values+nominals

– A knowledge base = ontology+instances

animal

rodent cowcat

mouse

eats

dog

domesticvermin

mickey

felix

jerry

tom

[Carole Goble, Nigel Shadbolt, Ontologies and the Grid Tutorial][Carole Goble, Nigel Shadbolt, Ontologies and the Grid Tutorial]

Light and Heavy expressivityLight and Heavy expressivity

• LightweightLightweight– Concepts, atomic types

– Is-a hierarchy

– Relationships between concepts

• HeavyweightHeavyweight– Metaclasses

– Type constraints on relations

– Cardinality constraints

– Taxonomy of relations

– Reified statements

– Axioms

– Semantic entailments

– Expressiveness

– Inference systems

A matter of rigour and representational expressivity

[Carole Goble, Nigel Shadbolt, Ontologies and the Grid Tutorial][Carole Goble, Nigel Shadbolt, Ontologies and the Grid Tutorial]

A semantic continuumA semantic continuum[Mike Uschold, Boeing Corp][Mike Uschold, Boeing Corp]

Shared human consensus

Text descriptions

Semantics hardwired; used at runtime

Semantics processed and used at runtime

Pump: “a device for moving a gas or liquid from one place or container to another”

(pump has (superclasses (…))

Implicit Informal(explicit)

Formal(for humans)

Formal(for machines)

Further to the right means: •Less ambiguity•More likely to have correct functionality•Better inter-operation (hopefully)

•Less hardwiring•More robust to change•More difficult

Requirements for Ontology LanguagesRequirements for Ontology Languages

• Ontology languages allow users to write explicit, formal Ontology languages allow users to write explicit, formal conceptualizations of domain modelsconceptualizations of domain models

• The main requirements are:The main requirements are:– a well-defined syntax – efficient reasoning support – a formal semantics – sufficient expressive power – convenience of expression

What have ontologies to do What have ontologies to do with the Semantic Web? with the Semantic Web?

The Semantic Web ApproachThe Semantic Web Approach

• Represent Web content in a form that is more easily Represent Web content in a form that is more easily machine-processablemachine-processable..

• Use intelligent techniques to take advantage of these Use intelligent techniques to take advantage of these representations. representations.

• The Semantic Web will The Semantic Web will gradually evolve out of the gradually evolve out of the existing Web, it isexisting Web, it is not a competition to the current WWW not a competition to the current WWW

The Semantic Web Layer TowerThe Semantic Web Layer Tower

The Role of Ontologies on the WebThe Role of Ontologies on the Web

• OOntologies provide a shared understanding of a domainntologies provide a shared understanding of a domain: : semantic interoperabilitysemantic interoperability– overcome differences in terminology

– mappings between ontologies

• Ontologies are useful for the organization and navigation Ontologies are useful for the organization and navigation of Web sites of Web sites

The Role of Ontologies in Web SearchThe Role of Ontologies in Web Search

• Ontologies are useful for improving the accuracy of Web searches Ontologies are useful for improving the accuracy of Web searches – search engines can look for pages that refer to a precise concept in an

ontology

• Web searches can exploit generalization/Web searches can exploit generalization/ specialization information specialization information – If a query fails to find any relevant documents, the search engine may

suggest to the user a more general query.

– If too many answers are retrieved, the search engine may suggest to the user some specializations.