Jarrar.lecture notes.aai.2011s.ontology part2_whatisontology

Lecture Notes University of Birzeit2nd Semester, 2010Advanced Artificial Intelligence (SCOM7341)OntologyPart 2 What is Ontology?Dr. Mustafa Jarrarmjarrar@birzeit.eduwww.jarrar.infoUniversity of Birzeit

Reading Material0) Everything in these slides1)Thomas R. Gruber: Toward Principles for the Design of Ontologies Used for Knowledge Sharing http://tomgruber.org/writing/onto-design.pdf2)Nicola Guarino: Formal Ontology and Information Systems http://www.loa-cnr.it/Papers/FOIS98.pdf

What is Ontology?In PhilosophyOntology 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 (كينونات) and the attributes (صفات) which belong to it qua being" (Aristotle, Metaphysics, IV, 1). It is the science of what is (in the universe) .Ontos (that which exists) + logos (knowledge of) Dates back to ArtistotleQuine, 1969: “To exist is to be the value of a quantified variable” So, it is a science (branch of philosophy).

What is Ontology?In computer scienceGruber (1995): “a explicit specification of a conceptualization”.the set of objects and relations in a domain<Objects, Relations, Functions>Written in logic, as a set of axioms i.e. a theoryConceptualization= <Objects, Relations, Functions>abdce

What is Ontology?In computer scienceGruber (1995): “a explicit specification of a conceptualization”.the set of objects and relations in a domain. <Objects,Relations,Functions>Written in logic, as a set of axioms i.e. a theoryConceptualization:Block {a, b, c, d, e}On {<a,b>,<b,c>,<d,e>}Above {<a,b>,<b,c>,<d,e>}Clear {<a>,<d>}Table {<c>,<e>}Hat {<b,a>,<c,b>,<e,d>}The ontology is a set of axioms used to specify this conceptualization: x y On(x,y)  Above(x,y) …abdceSharing these axioms (i.e., ontology) means sharing the same understanding

What is Ontology?In computer scienceGruber (1995): “a explicit specification of a conceptualization”.the set of objects and relations in a domain. <Objects,Relations,Functions>Written in logic, as a set of axioms i.e. a theoryGuarino’s: this change implies changing the conceptualization.

do we need to change our conceptualization each time there is some re- arrangements in the world?!Conceptualization:Block {a, b, c, d, e}On {<a,b>,<b,c>,<d,e>}Above {<a,b>,<b,c>,<d,e>}Clear {<a>,<d>}Table {<c>,<e>}Hat {<b,a>,<c,b>,<e,d>}adbce

What is Ontology?In computer scienceGruber (1995): “a explicit specification of a conceptualization”.the set of objects and relations in a domain. <Objects,Relations,Functions>Written in logic, as a set of axioms i.e. a theoryConceptualization:Block {a, b, c, d, e}On {<a,b>,<b,c>,<d,e>}Above {<a,b>,<b,c>,<d,e>}Clear {<a>,<d>}Table {<c>,<e>}Hat {<b,a>,<c,b>,<e,d>}Guarino’s: this conceptualization is a state of affairs (= one situation a snapshot) of the domain.

This definition of conceptualization has a problem.adbce

Guarino’s definition of a conceptualizationindependent of any specific interpretation, model, or situation,A concetualization is an intensional semantic structure, which encodes the implicit rules constraining the structure of a piece of realityThese should not be ordinary relations, but rather conceptual relations.

A relations has a model.(extensional interpretation).A conceptual relation has intended models.(Intensionalinterpretation).Conceptualization: [[Block]]D{a, b, c, d, e} [[On]]D{<a,b>,<b,c>,<d,e>} [[Above ]]D {<a,b>,<b,c>,<d,e>} [[Clear ]]D{<a>,<d>} [[Table ]]D{<c>,<e>}[[Hat ]]D{<b,a>,<c,b>,<e,d>}abdce

Guarino’s definition of a conceptualizationindependent of any specific interpretation, model, or situation,A concetualization is an intensional semantic structure, which encodes the implicit rules constraining the structure of a piece of realityOrdinary relations are defined on a domain D:Conceptual relations are defined on a domain space<D, W>An Ontology is an artifact designed with the purpose of expressing the intended meaning of a (shared) vocabulary.• A shared vocabulary plus a specification (characterization) of its intended meaning

Ontologies vs. Conceptual SchemasConceptual schemasOften not accessible at run timeUsually no formal semanticsattribute values taken out of the UoDconstraints relevant for database updateOntologiesUsually accessible at run timeformal semanticsattribute values first-class citizensconstraints relevant for intended meaning

Ontologies vs. Knowledge BasesKnowledge base – Assertional component• reflectsspecific (epistemic) states of affairs• designed for problem-solving – Terminological component (ontology)• independent of particular states of affairs• Designed to support terminological servicesOntological formulas are (assumed to be) necessarily true

Ontologies vs. classifications• Classifications focus on:– access, based on pre-determined criteria(encoded by syntactic keys)• Ontologies focus on:– Meaning of terms– Nature and structure of a domain

Is this an Ontology or a Data Schema?AddressHasPerson ⊑ HasAddress.String ⊓ hasEmailPersonEmailHasIn OWL<owl:Class rdf:ID=“Person" /><owl:Class rdf:ID=“Address" /><owl:Class rdf:ID=“email" /><owl:DataProperty rdf:ID=“Has-Address"> <rdfs:domain rdf:resource="#Person" /> <rdfs:range rdf:resource="www.w3.org/2001/XMLSchema#string"/></owl:ObjectProperty><owl:DataProperty rdf:ID=“Has-Email"> <rdfs:domain rdf:resource="#Person" /><rdfs:range rdf:resource="www.w3.org/2001/XMLSchema#string"/></owl:ObjectProperty> What makes and ontology an ontology

Educational InstitutionProjectparticipates-In/FacultiesComposed-Of/ Which of these characteristics are more distinguishing? (Intrinsic verse extrinsic characteristics)Example, What is X? where is the meaning/semantics?EmailHasAddressHasX“An intrinsic property (الصفات الجوهرية) is typically something inherent to an individual, not dependent on other individuals, such as having a heart or having a fingerprint. Extrinsic properties (الصفات العرضية) are not inherent, and they have a relational nature, like “being a friend of John”. Among these, there are some that are typically assigned by external agents or agencies, such as having a specific social security number, having a specific customer i.d., even having a specific name.”

Educational InstitutionProjectparticipates-In/FacultiesComposed-Of/Example, What is X? where is the meaning/semantics?EmailHasAddressHasXAn ontology that doesn’t hold intrinsic properties is not a good ontology, otherwise where is the semantics/real-meaning? (Ideally, it should“...catch all and only the intended meaning” [Gangemi 04])Notice that it is not necessary that the intrinsic properties be explicitly captured in the ontology, but these properties must govern the way we think and build the ontology.

(Some) Ontology Engineering Challenges Ontology Application DependenceOntologies are supposed to capture knowledge at the domain level independently of application requirements [G97] [GB99] [CJB99].The problem is that when building an ontology, there will always be intended or expected usability requirements -“at hand”- which influence the independency level of ontology axioms. Bylander and Chandrasekaran argued in [BC88] that:“Representing knowledge for the purpose of solving some problem is strongly affected by the nature of the problem and the inference strategy to be applied to the problem.”

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