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

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  • 1. Lecture Notes
    University of Birzeit
    2nd Semester, 2010
    Advanced Artificial Intelligence (SCOM7341)
    Ontology
    Part 2 What is Ontology?
    Dr. Mustafa Jarrar
    mjarrar@birzeit.eduwww.jarrar.info
    University of Birzeit
  • 2. Reading Material
    0) Everything in these slides
    1)Thomas R. Gruber: Toward Principles for the Design of Ontologies Used for Knowledge Sharing http://tomgruber.org/writing/onto-design.pdf
    2)Nicola Guarino: Formal Ontology and Information Systems http://www.loa-cnr.it/Papers/FOIS98.pdf
  • 3. What is Ontology?
    In Philosophy
    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 (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 Artistotle
    Quine, 1969: “To exist is to be the value of a quantified variable”
     So, it is a science (branch of philosophy).
  • 4. What is Ontology?
    In computer science
    • Gruber (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 theory
    Conceptualization
    = <Objects, Relations, Functions>
    a
    b
    d
    c
    e
  • 5. What is Ontology?
    In computer science
    Gruber (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 theory
    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>}
    The ontology is a set of axioms used to specify this conceptualization:
    x y On(x,y)  Above(x,y)

    a
    b
    d
    c
    e
    Sharing these axioms (i.e., ontology) means sharing the same understanding
  • 6. What is Ontology?
    In computer science
    Gruber (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 theory
    Guarino’s:
    • this change implies changing the conceptualization.
    • 7. 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>}
    a
    d
    b
    c
    e
  • 8. What is Ontology?
    In computer science
    Gruber (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 theory
    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>}
    Guarino’s:
    • this conceptualization is a state of affairs (= one situation a snapshot) of the domain.
    • 9. This definition of conceptualization has a problem.
    a
    d
    b
    c
    e
  • 10. Guarino’s definition of a conceptualization
    independent 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 reality
    • These should not be ordinary relations, but rather conceptual relations.
    • 11. 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>}
    a
    b
    d
    c
    e
  • 12. Guarino’s definition of a conceptualization
    independent 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 reality
    Ordinary 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
  • 13. Ontologies vs. Conceptual Schemas
    Conceptual schemas
    Often not accessible at run time
    Usually no formal semantics
    attribute values taken out of the UoD
    constraints relevant for database update
    Ontologies
    Usually accessible at run time
    formal semantics
    attribute values first-class citizens
    constraints relevant for intended meaning
  • 14. Ontologies vs. Knowledge Bases
    Knowledge 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 services
    Ontological formulas are (assumed to be) necessarily true
  • 15. 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
  • 16. Is this an Ontology or a Data Schema?
    Address
    Has
    Person ⊑ HasAddress.String
    ⊓ hasEmail
    Person
    Email
    Has
    In 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
  • 17. Educational Institution
    Project
    participates-In/
    Faculties
    Composed-Of/
    • Which of these characteristics are more distinguishing?
    (Intrinsic verse extrinsic characteristics)
    Example, What is X? where is the meaning/semantics?
    Email
    Has
    Address
    Has
    X
    “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.”
  • 18. Educational Institution
    Project
    participates-In/
    Faculties
    Composed-Of/
    Example, What is X? where is the meaning/semantics?
    Email
    Has
    Address
    Has
    X
    An 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.
  • 19. (Some) Ontology Engineering Challenges
    Ontology Application Dependence
    Ontologies 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.”
  • 20. (Some) Ontology Engineering Challenges
    ?
    Libraries
    Ontology Application Dependence
    Usability perspectives lead to different (and sometimes conflicting) axiomatizations although these axiomatizations might agree at the domain level.
    Bookstores
  • 21. (Some) Ontology Engineering Challenges
    ?
    Libraries
    Ontology Application Dependence
    Usability perspectives lead to different (and sometimes conflicting) axiomatizations although these axiomatizations might agree at the domain level.
    Bookstores
    Both are not ontologies, they are data schemes
  • 22. The Ontological Level
    [Guarino]