1. Introduction to OWL Introduction to OWL http://www.icsd.aegean.gr/kotis/OE&SW’07 http://www.icsd.aegean.gr/kotis/OE&SW’07 ΠΑΝΕΠΙΣΤΗΜΙΟ ΑΙΓΑΙΟΥ ΤΜΗΜΑ ΜΗΧΑΝΙΚΩΝ ΠΛΗΡΟΦΟΡΙΑΚΩΝ ΚΑΙ ΕΠΙΚΟΙΝΩΝΙΑΚΩΝ ΣΥΣΤΗΜΑΤΩΝ Πρόγραμμα Μεταπτυχιακών Σπουδών Κώτης Κων/νοςΚώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 Κώτης Κων/νος

2. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 2 Θέματα What is OWL Adds to RDF-S OWL versions (Lite, DL, Full) OWL Syntax Querying Part of Material was taken from Ian Horrocks lectures series  http://www.cs.man.ac.uk/~horrocks/Teaching/cs646/ http://www.cs.man.ac.uk/~horrocks/Teaching/cs646/

3. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 3 Ontology Languages Wide variety of languages for “Explicit Specification”  Graphical notations Semantic networks Topic Maps (see http://www.topicmaps.org/) UML RDF  Logic based Description Logics (e.g., OIL, DAML+OIL, OWL) Rules (e.g., RuleML, LP/Prolog) First Order Logic (e.g., KIF) Conceptual graphs Non-classical logics (e.g., F-logic)

4. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 4 Ontology Languages Wide variety of languages for “Explicit Specification”  Graphical notations Semantic networks

5. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 5 Ontology Languages Wide variety of languages for “Explicit Specification”  Graphical notations Topic Maps

6. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 6 Ontology Languages Wide variety of languages for “Explicit Specification”  Graphical notations UML

7. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 7 Ontology Languages Wide variety of languages for “Explicit Specification”  Graphical notations RDF

8. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 8 Ontology Languages Wide variety of languages for “Explicit Specification”  Logic based Description Logics (e.g., OIL, DAML+OIL, OWL) Rules (e.g., RuleML, LP/Prolog) First Order Logic (e.g., KIF)

9. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 9 Ontology Languages Wide variety of languages for “Explicit Specification”  Logic based Conceptual graphs

10. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 10 Objects/Instances/Individuals  Elements of the domain of discourse  Equivalent to constants in FOL Types/Classes/Concepts  Sets of objects sharing certain characteristics  Equivalent to unary predicates in FOL Relations/Properties/Roles  Sets of pairs (tuples) of objects  Equivalent to binary predicates in FOL Such languages are/can be:  Well understood  Formally specified  (Relatively) easy to use  machine processing Many languages use “object oriented” model based on:

11. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 11 Web “Schema” Languages Existing Web languages extended to facilitate content description  XML  XML Schema (XMLS)  RDF  RDF Schema (RDFS) XMLS not an ontology language  Changes format of DTDs (document schemas) to be XML  Adds an extensible type hierarchy Integers, Strings, etc. Can define sub-types, e.g., positive integers RDFS is recognisable as an ontology language  Classes and properties  Sub/super-classes (and properties)  Range and domain (of properties)

12. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 12 Problems with RDFS RDFS too weak to describe resources in sufficient detail  No localised range and domain constraints Can’t say that the range of hasChild is Person when applied to persons and Elephant when applied to elephants  No disjoint classes Can’t say that that male and female are disjoint  No union, intersection, complement of classes (boolean combinations of classes) Can’t say that class person is the disjoint union of classes male and female  No existence/cardinality constraints Can’t say that all instances of person have a mother that is also a person, or that persons have exactly 2 parents  No transitive, inverse or unique properties Can’t say that “grater than” is a transitive property, that hasPart is the inverse of isPartOf or that “is mother of” is unique property.

13. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 13 Example Schemas: a “university” description disjointWith equivalentClass Faculty OWL semantics Not allowed due to disjoint Classes

14. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 14 Web Ontology Language Requirements Desirable features identified for Web Ontology Language: Extends existing Web standards  Such as XML, RDF, RDFS Easy to understand and use  Should be based on familiar KR idioms Formally specified Of “adequate” expressive power Possible to provide automated reasoning support

15. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 15 Web Ontology Language Requirements Well defined syntax  machine processing of information …but not user-friendly RDF syntax is hard However, there are ontology developing tools that solve this problem…e.g. Protégé

16. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 16 Web Ontology Language Requirements Formal semantics = describes the meaning of knowledge precisely, i.e. No subjective intuitions, it is not open to different interpretations (by people or machines) Importance of formal semantics is large…it allows people to reason about knowledge!!!

17. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 17 Web Ontology Language Requirements For ontological knowledge: Class membership (if x is instance of class C, and C is subclass of D, then we can infer that x is an instance of D) Equivalence of classes (if class A is equivalent to class B, and class B is equivalent to class C, then A is equivalent to C, too) Consistency check (cannot A and B classes own both the instance x if these classes are disjoint) Automatic Classification of instances/concepts (conclude that x must be an instance of class A, if x satisfies certain property-value pairs conditions)

18. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 18 Web Ontology Language Requirements must” Semantics is a “must” for Reasoning Support. It allows one to: Check the consistency of the ontology and knowledge Check (not intended) relations between classes Automatically classify instances in classes

19. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 19 Web Ontology Language Requirements Formal semantics and reasoning are usually provided by mapping ontology language to a known logical formalism… OWL  Description Logic Use already existing reasoners, …FaCT, RACER. D.L support efficient reasoning

20. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 20 From RDF to OWL Two languages developed to satisfy above requirements  OIL: developed by group of (largely) European researchers (several from EU OntoKnowledge project)  DAML-ONT: developed by group of (largely) US researchers (in DARPA DAML programme) Efforts merged to produce DAML+OIL  Development was carried out by “Joint EU/US Committee on Agent Markup Languages”  Extends (“DL subset” of) RDF DAML+OIL submitted to W3C as basis for standardisation  Web-Ontology (WebOnt) Working Group formed  WebOnt group developed OWL language based on DAML+OIL  OWL language now a W3C Candidate Recommendation  Has become Proposed Recommendation

21. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 21 OWL Language Three species of OWL  OWL full is union of OWL syntax and RDF  OWL DL restricted to FOL fragment  OWL Lite is “easier to implement” subset of OWL DL OWL DL based on SHIQ Description Logic OWL DL benefits from many years of DL research  Well defined semantics  Formal properties well understood (complexity, decidability)  Known reasoning algorithms  Implemented systems (highly optimised)

22. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 22 OWL types OWL Lite supports those users primarily needing a classification hierarchy and simple constraint features completeness decidability OWL DL supports those users who want the maximum expressiveness without losing computational completeness (all entailments are guaranteed to be computed) and decidability (all computations will finish in finite time) of reasoning systems maximum expressivenesssyntactic freedom of RDF OWL Full is meant for users who want maximum expressiveness and the syntactic freedom of RDF with no computational guarantees

23. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 23 OWL types Each of these sublanguages is an extension of its simpler predecessor, both in what can be legally expressed and in what can be validly concluded. The following set of relations hold. Their inverses do not.  Every legal OWL Lite ontology is a legal OWL DL ontology.  Every legal OWL DL ontology is a legal OWL Full ontology.  Every valid OWL Lite conclusion is a valid OWL DL conclusion.  Every valid OWL DL conclusion is a valid OWL Full conclusion

24. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 24 OWL types Ontology developers adopting OWL should consider which species best suits their needs.  The choice between OWL Lite and OWL DL depends on the extent to which users require the more expressive restriction constructs provided by OWL DL.  Reasoners for OWL Lite will have desirable computational properties. Reasoners for OWL DL, while dealing with a decidable sublanguage, will be subject to higher worst-case complexity.  The choice between OWL DL and OWL Full mainly depends on the extent to which users require the meta- modelling facilities of RDF Schema (i.e. defining classes of classes).  When using OWL Full as compared to OWL DL, reasoning support is less predictable

25. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 25 What Are Description Logics? A family of logic based Knowledge Representation formalisms  Descendants of semantic networks and KL-ONE  Describe domain in terms of concepts (classes), roles (relationships) and individuals Distinguished by:  Formal semantics (typically model theoretic) Decidable fragments of FOL Closely related to Propositional Modal & Dynamic Logics  Provision of inference services Sound and complete decision procedures for key problems Implemented systems (highly optimised)

26. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 26 DL Architecture Knowledge Base Tbox (schema) Abox (data) Man ≡ Human ⊓ Male Happy-Father ≡ Man ⊓ ∃ has-child Female ⊓ … John : Happy-Father ( John, Mary ) : has-child Inference System Interface

27. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 27 Description Logic Family DLs are a family of logic based KR formalisms Particular languages mainly characterised by:  Set of constructors for building complex concepts and roles from simpler ones  Set of axioms for asserting facts about concepts, roles and individuals ALC is the smallest DL that is propositionally closed  Constructors include booleans (and, or, not), and  Restrictions on role successors

28. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 28 DL Concept and Role Constructors Range of other constructors found in DLs, including:  Number restrictions (cardinality constraints) on roles, e.g.,  3 hasChild,  1 hasMother  Qualified number restrictions, e.g.,  2 hasChild.Female,  1 hasParent.Male  Inverse roles, e.g., hasChild - (hasParent)  Transitive roles, e.g., hasChild* (descendant)  Role composition, e.g., hasParent o hasBrother (uncle)

29. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 29 DL Knowledge Base DL Knowledge Base (KB) normally separated into 2 parts:  TBox is a set of axioms describing structure of domain (i.e., a conceptual schema), e.g.: HappyFather ≡ Man ⊓  hasChild.Female ⊓ … Elephant ≡ Animal ⊓ Large ⊓ Grey transitive(ancestor)  ABox is a set of axioms describing a concrete situation (data), e.g.: John:HappyFather :hasChild Separation has no logical significance  But may be conceptually and implementationally convenient

30. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 30 OWL as DL: Class Constructors

31. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 31 OWL example in RDFS Syntax E.g., Person ⊓ ∀ hasChild.Doctor ⊔ ∃ hasChild.Doctor:

32. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 32 OWL as DL: Axioms

33. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 33 OWL DL Semantics Mapping OWL to equivalent DL:  Facilitates provision of reasoning services (using DL systems)  Provides well defined semantics DL semantics defined by interpretations: I =(  I,. I ), where   I is the domain (a non-empty set) . I is an interpretation function that maps: Concept (class) name A →subset A I of  I Role (property) name R → binary relation R I over  I Individual name i → i I element of  I

34. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 34 DL Knowledge Bases (Ontologies) An OWL ontology maps to a DL Knowledge Base  T (Tbox) is a set of axioms of the form: C ⊑ D (concept inclusion) C ≡ D (concept equivalence) R ⊑ S (role inclusion) R ≡ S (role equivalence) R + ⊑ R (role transitivity)  A (Abox) is a set of axioms of the form X ∊ D (concept instantiation) (x,y) ∊ R (role instantiation)

35. Reasoning with OWL

36. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 36 Why do we want/need to reason with OWL? Semantic Web aims at “machine understanding” Understanding closely related to reasoning  Recognising semantic similarity in spite of syntactic differences  Drawing conclusions that are not explicitly stated 1. Philosophical Reasons

37. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 37 2. Practical Reasons Given key role of ontologies in e-Science and Semantic Web, it is essential to provide tools and services to help users:  Design and maintain high quality ontologies, e.g.: Meaningful — all named classes can have instances Correct — captured intuitions of domain experts Minimally redundant — no unintended synonyms Richly axiomatised — (sufficiently) detailed descriptions  Store (large numbers) of instances of ontology classes, e.g.: Annotations from web pages (or gene product data)  Answer queries over ontology classes and instances, e.g.: Find more general/specific classes Retrieve annotations/pages matching a given description  Integrate and align multiple ontologies

38. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 38 Basic Inference Tasks

39. OWL Guide/Reference

40. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 40 OWL Syntax OWL still uses RDF and RDF Schema to a large extent:  All 3 types of OWL use RDF for syntax  Instances are declared as in RDF, using RDF descriptions  OWL constructors like owl:Class, owl:ObjectProperty, owl:DatatypeProperty are specializations of RDF

41. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 41 OWL Syntax Subclass relationship between OWL and RDF/RDFS rdfs:Resource rdfs:Class owl:Class rdf:Property owl:ObjectPropertyowl:DatatypeProperty

42. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 42 OWL Syntax 4 different forms  RDF/XML syntax  XML-based syntax (more easy for humans)  Abstract syntax (more readable)  Graphic syntax based on UML (human oriented) We use the first since it is the primary syntax for OWL

43. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 43 Header - namespaces <rdf:RDF xmlns:owl ="http://www.w3.org/2002/07/owl#" xmlns:rdf ="http://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#" xmlns:xsd ="http://www.w3.org/2001/XMLSchema#"> OWL ontologies RDF documents

44. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 44 “housekeeping”info Imports content from other ontologies to be Part of the current ontology Reminder: Namespaces are used for disambiguation of resources Imported ontologies provide definitions that can be used

45. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 45 Define Classes Classes are defined with owl :Class element Subsumption relation ( subClassOf )  E.g. Borrows element from RDFSchema

46. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 46 Define Classes Classes are defined with owl :Class element Disjoint relation ( disjointWith )  E.g. OWL element

47. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 47 Define Classes Classes are defined with owl :Class element Equivalence relation ( equivalentClass )  E.g. OWL element

48. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 48 Predefined Classes - Enumeration Predefined Classes are owl:Thing and owl:Nothingowl:Thingowl:Nothing  class extension of owl:Thing is the set of all individuals  The class extension of owl:Nothing is the empty set  Every OWL class is a subclass of owl:Thing and owl:Nothing is a subclass of every class A class description of the "enumeration" kind is defined with the owl:oneOf property. The value of this built-in OWL property must be a list of individuals that are the instances of the class.owl:oneOfinstances

49. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 49 Predefined Classes - Enumeration A class can be described by exhaustively enumerating its instances.  The class extension of a class described with owl:oneOf contains exactly the enumerated individuals, no more, no less.

50. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 50 Define Classes – boolean combinations Intersection constructor ( intersectionOf)  An owl:intersectionOf statement describes a class for which the class extension contains precisely those individuals that are members of the class extension of all class descriptions in the list. = “Tosca”

51. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 51 Define Classes – Boolean combinations union constructor (union Of)  An owl:unionOf statement describes an anonymous class for which the class extension contains those individuals that occur in at least one of the class extensions of the class descriptions in the list. = Tosca, Salome, Turandot

52. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 52 Define Classes – Boolean combinations More examples

53. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 53 Define Properties Properties are defined with owl :ObjectProperty and owl :DatatypeProperty elements  ObjectProperty relates objects to objects E.g. Course isTaughtBy academicStaff  DatatypeProperty relates objects to datatype values E.g. Age nonNegativeInteger

54. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 54 Define Properties DatatypeProperty relates objects to datatype values E.g. Age nonNegativeInteger Reminder: Domain/Range= restrictions on the values of properties e.g.Knowledge Repr. is taught by Semantic Web (no sense) we need to restrict the range of the property Room B5 is taught by K. Kotis (no sense) we need to restrict the domain of the property

55. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 55 Define Properties ObjectProperty relates objects to objects E.g. Course isTaughtBy academicStaff CourseacademicStaff isTaughtBy domain range involves Staff

56. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 56 Define Properties owl: inverseOf defines the inverse property of a property E.g. teaches / isTaughtBy CourseacademicStaff teaches domain range isTaughtBy range domain

57. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 57 Define Properties owl: equivalentProperty defines the equivalent of a property E.g. teaches / lecturesIn

58. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 58 Property restrictions taught by E.g. : “First year Courses must be taught by Professors only” Note: the subClassOf is used to declare that if a class C satisfies certain conditions, THEN all instances of C satisfy the conditions. This is equivalent to “C subClassOf D”, where D collects all objects that satisfy the conditions

59. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 59 Property restrictions teach E.g. : “All academic Staff must teach at least one undergraduate Course”

60. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 60 Property restrictions taught by E.g. : “Every Course must be taught by at least someone” 1

61. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 61 Property restrictions E.g. : “A Department must have at least 10 and as most 30 members” 10 30

62. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 62 Property restrictions E.g. : “A Phd Student must have exactly 3 supervisors” 3 3 Use also owl :cardinality

63. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 63 Special logical Properties TransitiveProperty Regions E.g. :“the subRegionOf property between Regions is transitive” If A,B,C are Regions, and “A subRegionOf B” and “B subRegionOf C”, THEN the reasoner will be able to derive that “A also subRegionOf C”. NOTE: OWL DL requires that for a transitive property no local or global cardinality constraints should be declared on the property itself or its superproperties, nor on the inverse of the property or its superproperties.

64. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 64 Special logical Properties SymmetricProperty Humans E.g. :“the friendOf property between Humans is symmetric” If A,B are Humans, and “A friendOf B” THEN the reasoner will be able to derive that “B also friendOf A”.

65. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 65 Instances (Individuals) In addition to classes, we want to be able to describe their members. We think of these as individuals in our universe of things. An individual is minimally introduced by declaring it to be a member of a class. Individuals are defined with individual axioms (also called "facts"). We discuss two types of facts:  Facts about class membership and property values of individuals  Facts about individual identity

66. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 66 Instances -Class membership and property values Many facts typically are statements indicating class membership of individuals and property values of individuals  E.g. an instance of the class Opera 1900-01-14 3..includes a number of facts about the individual Tosca, an instance of the class Opera 1 1 ….

67. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 67 Instances -Class membership and property values Subclass vs. instance  It is very easy to confuse the instance-of relationship with the subclass relationship.  It may seem arbitrary to choose to make Tosca an individual that is an instance of Opera, as opposed to a subclass of Opera. This is not an arbitrary decision. Tosca  The Opera class denotes the set of all opera varietals, and therefore any subclass of Opera should denote a subset of these varietals. Thus, Tosca should be considered an instance of Opera, and not a subclass. It does not describe a subset of Opera varietals, it is an opera varietal

68. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 68 Instances -Class membership and property values E.g. wine ontology  Instance of WineGrape

69. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 69 Instances - individual identity Many languages have a so-called "unique names" assumption: different names refer to different things in the world On the web, such an assumption is not possible  E.g. the same person could be referred to in many different ways (i.e. with different URI references)  OWL does not make this assumption Unless an explicit statement is being made that “two URI references refer to the same or to different individuals”, OWL tools should in principle assume either situation is possible.

70. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 70 Instances - individual identity OWL provides three constructs for stating facts about the identity of individuals:  owl:sameAs is used to state that two URI references refer to the same individual.  owl:differentFrom is used to state that two URI references refer to different individuals  owl:AllDifferent provides an idiom for stating that a list of individuals are all different.

71. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 71 Instances - individual identity owl:sameAs is used to state that two URI references refer to the same individual…: In OWL Full, where a class can be treated as instances of (meta)classes, we can use the owl:sameAs construct to define class equality, thus indicating that two concepts have the same intensional meaning. An example: states that the two classes have the same class extension, but are not (necessarily) the same concepts.

72. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 72 Instances - individual identity owl:differentFrom is used to state that two URI references refer to different individuals  An example: This states that there are three different operas.

73. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 73 Instances - individual identity owl:AllDifferent provides an idiom for stating that a list of individuals are all different.  An example: This states that these six URI references all point to different operas.

74. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 74 Examples - Ontologies http://www.schemaweb.info/schema/BrowseSchema.aspx http://protege.stanford.edu/plugins/owl/ontologies.html

75. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 75 Data Types we have seen the notion of a data range for specifying a range of data values. OWL allows three types of data range specifications:  An RDF datatype specification.RDF datatype  The RDFS class rdfs:Literal.rdfs:Literal  An enumerated datatype, using the owl:oneOf construct.enumerated datatype

76. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 76 Data Types When using datatypes, please note that even if a property is defined to have a range of a certain datatype, RDF/XML still requires that the datatype be specified each time the property is used.  example: 2003-01- 24T09:00:08+01:00

77. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 77 Data Types OWL provides one additional construct for defining a range of data values, namely an enumerated datatype. This datatype format makes use of the owl:oneOf construct, that is also used for describing an enumerated class.enumerated class

78. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 78 Importing an ontology An owl:imports statement references another OWL ontology containing definitions, whose meaning is considered to be part of the meaning of the importing ontology.owl:imports  Each reference consists of a URI specifying from where the ontology is to be imported. The owl:imports statements are transitive  if ontology A imports B, and B imports C, then A imports both B and C. Although owl:imports and namespace declarations may appear redundant, they actually serve different purposes.  Namespace declarations simply set up a shorthand for referring to identifiers. They do not implicitly include the meaning of documents located at the URI.  owl:imports does not provide any shorthand notation for referring to the identifiers from the imported document.

79. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 79 Version information An owl:versionInfo statement generally has as its object a string giving information about this versionowl:versionInfo Although this property is typically used to make statements about ontologies, it may be applied to any OWL construct.  For example, one could attach a owl:versionInfo statement to an OWL class.

80. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 80 Layering of OWL OWL Full: all constructors can be used  As long as the result is legal RDF OWL DL  any resource allowed to be only: class, data type, data type property, object property, individual, data value, part of the built-in vocabulary, and not more than one of these  e.g. a class cannot be an individual at the same time!!!  Explicit typing, i.e. nothing is assumed, must be declared!!!  Set of object properties and data type properties are disjoint. E.g. cannot specify owl:inverseOf for data type property.  No cardinality restrictions may be placed on transitive properties OWL Lite  Do not allow the constructors: (owl:) one of, disjointWith, unionOf, complementOf, hasValue  Cardinality only allowed for values 0 and 1  owl:equivalentClass is only allowed between class identifiers, not anonymous classes

81. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 81 Discussion The idea behind the OWL Lite expressivity limitations is that they provide a minimal useful subset of language features, that are relatively straightforward for tool developers to support. The language constructs of OWL Lite provide the basics for subclass hierarchy construction: subclasses and property restrictions. OWL Lite allows properties to be made optional or required. The limitations on OWL Lite place it in a lower complexity class than OWL DL. This can have a positive impact on the efficiency of complete reasoners for OWL Lite. Implementations that support only the OWL Lite vocabulary, but otherwise relax the restrictions of OWL DL, cannot make certain computational claims with respect to consistency and complexity. However, such implementations may be useful in providing interoperability of OWL systems with RDFS models, databases, markup tools, or other non-reasoning tools. The Web Ontology Working Group has not provided a name for this potentially useful subset.

82. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 82 Examples - Ontologies http://www.schemaweb.info/schema/BrowseSchema.aspx http://protege.stanford.edu/plugins/owl/ontologies.html Do not forget that everything about OWL can be found at http://www.w3c.org http://www.w3c.org

83. 27/10/2015 Κώτης Κων/νος - Copyright Ai-Lab, ICSEng. Dept. University of the Aegean - 2007 83 Querying OWL-QL (under recommendation) http://ksl.stanford.edu/projects/owl-ql/ http://ksl.stanford.edu/projects/owl-ql/  E.g. click hereclick here Ontology Tools e.g. Protégé querying system  E.g.