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).

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

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

9. This definition of conceptualization has a problem.a d b 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

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]