Ontology Design: OWL Constructs. Tutor: Aldo Gangemi Lecture LEX09 Fiesole, Italy

Transcription

1 Ontology Design: OWL Constructs Tutor: Aldo Gangemi Lecture LEX09 Fiesole, Italy

2 Name: Aldo Gangemi Tutor info Institute: ISTC-CNR, Rome, Italy Research Group: Semantic Technology Laboratory (STLab) Role: Senior Researcher, Head of STLab Research interest: Ontology Design, KE+NLP, Enterprise 3.0, Legal Ontologies, Historical Knowledge Representation LEX09

3 The Web evolves A digital library (1.0) A document library with hyperlinks A database, an application platform (1.0, evolved) A shared portal through which applications can be web accessible A multimedia and social platform (2.0) A radio, a trailer, a telephone, a tv, a public place, a position anywhere in the world. Collaboration, web editing A naming scheme (Semantic Web) Unique identity for all entities: documents, persons, services, physical things, metadata,... URI-based data integration is the key use-case for the semantic web A Global Giant Graph (3.0) A place where people work, interact, and produce interpretations assisted by computers 3

4 Use URIs as names for things Data patterns Use HTTP URIs so that people can look up those names Slash URIs are preferred from &m berners lee s linked data principles When someone looks up a URI, provide useful information Browsable graphs: a graph G is browsable if, for the URI of any node in G, if I look up that URI I will be returned information which describes the node, where describing a node means: Returning all statements where the node is a subject or object Describing all blank nodes attached to the node by one arc Include links to other URIs so that they can discover more things 4

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6 RDF vocabularies and OWL Large use of small vocabularies FOAF, SIOC, SKOS OWL representation and reasoning is the next step at a web scale, but already a reality for intrawebs 6

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8 Preliminaries Team in pairs or small groups Open an editor Create an ontology project Choose the reference language or syntax Create a new ontology Choose the base URI of the ontology ( ID ), and the default URI (ending with either # or / ) 8

9 Natural and Formal Languages Formal interpretation is not (only :)) an academic game It gives us a precise way to establish what we are talking about, and therefore to provide reliable automated inferences when needed Natural language is able to describe very different types of facts with the same structure, for example... 9

10 ... different types of facts... Melville wrote Moby Dick [ground fact] Captain Ahab is in command of the Pequod [literary fact] Moby Dick is white [attributive fact] The Pequod is a whaler [classification fact] A whaler is a ship for hunting whales [meaning fact] Whaler is synonym to whaleship [terminological fact] Whaler is six characters long [information fact] Whaler is a class [formal fact] Whales are a delicious food for Japanese people [contextual fact] Moby Dick represents the hatred and rage of humanity [interpretive fact] 10

11 From NL to FL This functionality of natural language cannot be easily reproduced for machine interpretation The formal interpretation of OWL can do something More is provided by best practices Some arbitrariness is unavoidable Requirements, requirements, requirements 11

12 RDF (and OWL) Each entity in an ontology is a web resource, and has a URI Each fact can be expressed as a triple > Let's create a new ontology Moby Dick 12

13 Facts in OWL (or RDF) Basic structure Subject Predicate Object > Melville wrote Moby Dick > Moby Dick is a whale 13

14 Embedding facts (RDF) If the fact includes three entities, you can embed a triple in another Whales are a delicious food for Japanese people [Whales are a delicious food] for Japanese people This is not allowed in OWL Patterns do exist to approximate such cases 14

15 Liberality of RDF As a matter of fact, you can declare any fact in RDF, even unusual, counterintuitive, or abnormal ones:? Moby Dick is white but also black? Moby Dick is in command of the Pequod? Ahab hates the third character of the white whale? Ahab is a class Liberality is great, but has costs: no way to make a machine detect undesired facts 15

16 OWL constraints In order to limit (and to guide) the design of ontologies, OWL restricts the expressivity of RDF No more is any triple allowed, but only those that respect the constraints of OWL formal semantics (lecture 1) Undesired facts will then be detected, if the design of the ontology reflects the conceptualization of the users That s why we need good design (lecture 2) 16

17 Formal interpretation (extensional) Whale MobyDick hates I = Human I Thing I Whale I Thing I Human I Thing I Whale I Human I = Ahab I Human I MobyDick I Whale I (Ahab,MobyDick) I hates I Ahab Human Thing hates 17

18 OWL exercise I: types, subclasses, and inheritance > Ahab hates Moby Dick > Ahab is a human > Moby Dick is a whale > Whale is a class > Whale is a subclass of Mammal > Mammal is a subclass of Vertebrate > Vertebrate is a Group 18

19 Inheritance Notice that some inferences are now allowed Moby Dick is a whale; whales are mammals Moby Dick is a mammal Whale is a subclass of Mammal; Mammal is a subclass of Vertebrate Whale is a subclass of Vertebrate This is called inheritance 19

20 Prevented inheritance But some inferences are prevented Moby Dick is a whale; Whale is an (OWL) class Moby Dick is an (OWL) class Mammal is a subclass of Vertebrate; Vertebrate is a group Mammal is a group 20

21 Lesson learnt In OWL we must distinguish when is a means rdf:type, and when it means rdfs:subclassof There are patterns to deal with these differences OWL2 allows more freedom 21

22 OWL exercise II: disjointness and consistency use Radon or Pellet to check the ontology after each addition > Whales are not humans In formal languages, we can usually state when two classes are disjoint This provides greater clarity, and works fine for formal checking, i.e. coherency or consistency 22

23 Consistency Equipped with the owl:disjointwith axiom, we can now exclude the following: Moby Dick is a human Ahab is a whale Since they would make an ontology inconsistent 23

24 Coherence We can also exclude the following: Whamans are whales and humans Since it would make the ontology incoherent 24

25 OWL exercise III: domain, ranges, inverses, subproperties > Whales can be food for Japanese humans > Being able to be food for something implies being able to be fed by something > Moby Dick can be food for Yukio > Hating something implies disliking it 25

26 Materialization Some new inferences are now allowed: Moby Dick can be food for Yukio Yukio can be fed by Moby Dick Ahab hates Moby Dick Ahab dislikes Moby Dick This is called materialization 26

27 Classification Yukio can be fed by Moby Dick; (as far as we know) only Japanese humans can be fed by whales Yukio is a Japanese human This is called classification But this is prevented: Yukio can be food for Moby Dick Why? 27

28 Open World Assumption Since the Web is an open world, if we say something that is not explicitly put in our axioms, we cannot exclude it, and then we have to add a new axiom: Yukio can be food for Moby Dick; whales can be food for Japanese humans Yukio is also a whale, and Moby Dick is also a Japanese human But, if we had modelled JapaneseHuman as a subclass of Human, this generates an inconsistency, since Human and Whale are disjoint classes, and JapaneseHuman is also disjoint with Whale, because of inheritance 28

29 OWL exercise IV: sameness and difference Due to the open world assumption, it is important to explicitly declare if any two individuals are the same or are different (when this is known) > Moby Dick is the same as The White Whale > Ahab is different from Yukio 29

30 Advanced modelling: restrictions and definitions in OWL The formal semantics of OWL is currently based on so called description logics. An important feature of description logics is the ability to declare unnamed constructs that are interpreted as classes. > Let's create a new ontology Aquatic organisms 30

31 OWL exercise V: boolean class constructors The class of things that are mammals and aquatic organisms The class including aquatic mammals, fishes and crustaceans The class of things that are not fishes not(fish) AquaLc Organism Mammal Fish AquaLc Mammal Crustacean Fish 31

32 OWL exercise VI: relational class constructors The class of things that only live in a marine habitat livein.marinehabitat The class of things that live in at least one marine habitat livein.marinehabitat The class of things that live in the Indian Ocean livein.{indianocean} The class of things that live in either the Indian or Pacific Ocean livein.{indianocean PacificOcean} The class of things that have taxonomic species Monodon monoceros taxonomicspecies.{monodonmonoceros} 32

33 OWL exercise VII: enumerating class constructors The class of the following things: Indian, Pacific, and Artlantic Oceans {IndianOcean PacificOcean AtlanticOcean} 33

34 OWL exercise VII: enumerating class constructors The class of the following things: Indian, Pacific, and Artlantic Oceans {IndianOcean PacificOcean AtlanticOcean} 33

35 OWL exercise VIII equivalence and automated subsumption/classification Differently from other conceptual modeling languages, OWL allows to state formal equivalence between two classes Equivalence works implicitly with class constructors ontology APIs generate internal classes that are equivalent to class constructors but equivalence can be stated explicitly.

36 ... continued... > Aquatic mammals are mammals and aquatic organisms > Aquatic organisms only include aquatic mammals, fishes and crustaceans > Whales are not fishes > Omnivore animals are animals that eat any other animals or plants > Carnivore animals are animals that only eat other animals, and eat some of them

37 ... continued... > Aquatic mammals are mammals and aquatic organisms > Aquatic organisms only include aquatic mammals, fishes and crustaceans > Whales are not fishes > Omnivore animals are animals that eat any other animals or plants > Carnivore animals are animals that only eat other animals, and eat some of them

38 Fuzzy OWL Not for all concepts or not in all contexts/ applications it is possible to assume crisp conditions, under which something is e.g. a legal subject, or an omnivore animal If a user needs to represent fuzzy concepts, should use a fuzzy variety of OWL But compare necessary conditions, sufficient conditions, scope of the ontology, and real fuzziness 36

39 ... cont.... Necessary condition on OmnivoreAnimal Omnivore animals eat any other animals or plants Sufficient condition to OmnivoreAnimal Animals that eat any other animals or plants are omnivore animals Definition of OmnivoreAnimal Omnivore animals are animals that eat any other animals or plants Fuzzy condition on OmnivoreAnimal Omnivore animals, under certain (but not necessarily all) relevant circumstances, eat any other animal or plant 37

40 OWL exercise IX symmetry and transitivity Object properties can be symmetric, or transitive. Sibling is an example of both symmetric and transitive object property

41 ... continued... > siblingorder and siblingspecies are symmetric and transitive > Cetacea and Artyodactyla are sibling orders of superorder Laurasiatheria > Within Cetacea, Monodon monoceros is a sibling species of Orcynus Orca, which is a sibling species of Balaenoptera musculus

42 ... continued... > siblingorder and siblingspecies are symmetric and transitive > Cetacea and Artyodactyla are sibling orders of superorder Laurasiatheria > Within Cetacea, Monodon monoceros is a sibling species of Orcynus Orca, which is a sibling species of Balaenoptera musculus

43 OWL exercise X datatype and annotation properties When dealing with XSD datatypes, OWL uses Datatype Properties > Moby Dick is 23 metres long When dealing with annotations, OWL uses Annotation Properties > Moby Dick has been inspired by a long hunting for the albino sperm whale Mocha Dick near the coast of Chile (more than 100 battles with whalers) 40

44 OWL exercise XI imports Everything that is true of Cetaceans ontology is true for Moby Dick ontology

45 OWL exercise XI imports Everything that is true of Cetaceans ontology is true for Moby Dick ontology

46 Abstract Syntax OWL Syntaxes Used in the definition of the language and the DL/Lite semantics OWL in RDF (the official concrete syntax) RDF/XML presentation XML Presentation Syntax XML Schema definition Other, Human readable syntaxes Manchester OWL Syntax Sydney OWL Syntax Rabbit borrowed from Sean Bechhofer s SSSW08 slides 42

47 Common Misconceptions Disjointness of primitives Interpreting domain and range And and Or Quantification Closed and Open Worlds borrowed from Sean Bechhofer s SSSW08 slides 43

48 Disjointness By default, primitive classes are not disjoint. Unless we explicitly say so, the description (Animal and Vegetable) is not inconsistent. Similarly with individuals -- the so-called Unique Name Assumption (often present in DL languages) does not hold, and individuals are not considered to be distinct unless explicitly asserted to be so. borrowed from Sean Bechhofer s SSSW08 slides 44

49 Domain and Range OWL allows us to specify the domain and range of properties. Note that this is not interpreted as a constraint. Rather, the domain and range assertions allow us to make inferences about individuals. Consider the following: ObjectProperty: employs Domain: Company Range: Person Individual: IBM Facts: employs Jim If we haven t said anything else about IBM or Jim, this is not an error. However, we can now infer that IBM is a Company and Jim is a Person. borrowed from Sean Bechhofer s SSSW08 slides 45

50 And/Or and Quantification The logical connectives And and Or often cause confusion Tea or Coffee? Milk and Sugar? Quantification can also be contrary to our intuition. Universal quantification over an empty set is true. Sean is a member of haschild only Martian Existential quantification may imply the existence of an individual that we don t know the name of. borrowed from Sean Bechhofer s SSSW08 slides 46

51 OWL 2 Several new features, two are especially important Property chains Mustafa is brother of Saida; Saida is mother of Rashid Mustafa is uncle of Rashid Multiple interpretations of a constant Tutor(Meri) AllowedPersonnel(Tutor) Tutor(Alonzo,Meri) 47