2. Knowledge Representation Applied Artificial Intelligence

Transcription

1 2. Knowledge Representation Applied Artificial Intelligence Prof. Dr. Bernhard Humm Faculty of Computer Science Hochschule Darmstadt University of Applied Sciences 1

2 Retrospective Introduction to AI What does Artificial Intelligence mean? Sketch the history of AI In what sense is AI today relevant and ubiquitous? Name a few prominent AI projects What are main areas of AI? What are relevant questions / technologies? 2

3 Agenda Overview RDF Arts Ontology 3

4 Knowldege to be represented Example: Arts Michelangelo was an Italian artist He created many paintings, e.g., the Creation of Adam in the Sistine Chapel, Rome He also created sculptures like David He belonged to the artistic movement of High Renaissance People who create paintings are painters Painters are artists Individuals Relationships Classes Rules 4 Image source: wikimedia

5 Ontology Knowldege represented for a particular application domain, e.g., arts Contains: - Schema: class types (e.g., person ) and relationship types (e.g., has painted - Facts: individuals (class instances, e.g., Michelangelo ) and their relationships (e.g, Michelangelo painted Creation of Adam ) Is formalized with a particular knowledge reprentation approach / language 5

6 Knowledge Representation approaches Predicate Logic painted (Michelangelo, CreationOfAdam) p: ( x: painted (p, x)) painter (p) Frames Michelangelo Type: Person born: 1475 in: Italy CreationOfAdam type: Painting artist:michelangelo Semantic Nets Michelangelo born-in Italy created CreationOfAdam Rules (?p instance-of person) (?p painted?x) --> (?p instance-of painter) 6

7 Knowledge representation and reasoning Known (asserted facts and rules) Michelangelo is a person Michelangelo has created the painting Creation of Adam Every person that has created a painting is a painter Derived (via reasoning) Michelangelo is a painter 7

8 Knowledge Representation and Querying Who has painted Creation of Adam? Michelangelo Has Michelangelo painted Creation of Adam? Yes Find all painters Michelangelo, 8 Requires Reasoning!

9 Open vs. Closed World Assumption Find all painters Michelangelo, Raffael, Leonardo da Vinci, Dürer, Rembrandt, Picasso, Magritte. 9 Are these all? Are you sure? Closed Word Assumption (CWA): - The database (asserted and inferred facts) is complete - Everything that cannot be found or inferred is not existing (wrong) - Negation as failure, e.g. Is Rembrandt a scupltur? No, since we don t know a sculpture of his Open World Assumption (OWA): - Facts can be added to the database at all times - Never say never

10 Agenda Overview RDF Arts Ontology 10

11 Resource Description Framework (RDF) Knowledge representation language Partially based on predicate logic and semantic nets Specified by World Wide Web Consortium (W3C) as part of the Semantic Web initiative Further W3C specifications: - RDF Schema (RDFS) - Web Ontology Language (OWL) 11

12 Resources and URIs Resource: anything that someone might want to talk about, e.g. - Michelangelo - All painters - Michelangelo s date of birth Every resource is has Uniform Resource Identifier (URI), e.g. < 12

13 Namespaces Abbreviation scheme: qnames with namespaces Namespace prefix dbpedia: < dbpedia:michelangelo Prefix usage Resource 13

14 Some common namespaces Name-space Description URI rdf: rdfs: owl: xsd: dbpedia: dbpedia-owl dbpprop yago Resource Description Framework RDF Schema Web Ontology Language XML Schema Dbpedia Resource Dbpedia Ontology Dbpedia Property YAGO Ontology < < < < 14

15 Triples: the basic statement notation of RDF Statement including subject, predicate, and object Subject: resource Predicate: resource Object: resource or literal value (XML data type) Examples: Subject Predicate Object: String Literal (English) Object: XML Schema Date dbpedia:michelangelo rdfs:label dbpedia:michelangelo dbpedia-owl:birthdate " :00"^^xsd:date. dbpedia:michelangelo dbpedia-owl:movement dbpedia:renaissance. Object: Resource dbpedia:michelangelo rdfs:label dbpedia-owl:birthdate dbpedia-owl:movement "Michelangelo" " " dbpedia:renaissance 15

16 Abbreviation and graphical semantic net notation Semicolon chains multiple triples with identical subject dbpedia:michelangelo rdfs:label ; dbpedia-owl:birthdate " :00"^^xsd:date ; dbpedia-owl:movement dbpedia:renaissance. dbpedia:michelangelo rdfs:label dbpedia-owl:birthdate dbpedia-owl:movement "Michelangelo" " " dbpedia:renaissance 16

17 Classes and Instances: rdf:type and rdfs:class rdf:type expresses instance relationship rdfs:class represents a meta-class Examples: Abbreviation for rdf:type : a Is-a relationship between individual and class: Michelangelo is a person dbpedia:michelangelo rdf:type dbpedia-owl:person. dbpedia-owl:person rdf:type rdfs:class. Person is a class Schema information (class definitions) with the same language constructs as facts. dbpedia:michelangelo a dbpedia-owl:person. dbpedia-owl:person a rdfs:class. 17

18 Classes / Instances in RDF(S) vs. Object-Orientation Concept of class (group) and instances (individuals) in Semantic Web comparable with Object-Orientation But there are major differences: Object-Orientation No instance without class Instance belongs to exactly one class Class specifies attributes and methods of all instances Semantic Web Any resource can be used no need for specifying a class and an rdf:type relationship Zero, one, or many rdf:type relationships may be specified No restrictions on the relationships of instances whatsoever. Methods have no equivalent at all 18

19 Properties rdf:property - Similar to rdfs:class - Declares a Property that can be used as a predicate - Example: Movement is a property dbpedia-owl:movement rdf:type rdf:property. dbpedia:michelangelo dbpedia-owl:movement dbpedia:renaissance. 19

20 No Type Checking! There is no type checking for subjects, predicates and objects! Example: dbpedia:michelangelo dbpedia-owl:birthdate "Nonsense". dbpedia:michelangelo dbpedia:michelangelo dbpedia:michelangelo. No mechanism prevents such a nonsense! Why? Semantic Web principle: Anyone can say anything about any topic (similar to classic web pages) 20

21 Properties in RDF vs. Object-Orientation Properties comparable with associations between class instances But there are major differences: Object-Orientation Associations between instances are specified in class definition Types of association ends are specified Type violations result in errors (compile time or run-time) Semantic Web No need for specifying rdf:type rdf:property No need for specifying rdf:domain and rdf:range No type checking (neither compile time nor run-time) Instead: Reasoning results may be not as expected 21

22 Serialization syntaxes for RDF Turtle / N3 (most convenient; used in this course): dbpedia:michelangelo dbpedia-owl:movement dbpedia:renaissance. N-Triples < < < RDF /XML <rdf:description rdf:about=" <dbpedia-owl:movement> </dbpedia-owl:movement> </rdf:description> 22

23 Literature Dean Allemang, James Hendler: Semantic Web for the Working Ontologist: Effective Modeling in RDFS and OWL. Morgan Kaufmann Publishers Inc., San Francisco. 23

24 Agenda Overview RDF Arts Ontology 24

25 Arts Ontology in Protege Arts Ontology Custom subset of Dbpedia 4 MB ca paintings 200 sculptures 400 artists 200 museums 80 artistic movements 800 locations 25

26 Dbpedia: Linked open data set based on Wikipedia 26 Image source: wikimedia

27 Arts Ontology Schema Location +dbpedia-owl:ispartof Person +dbpedia-owl:influencedby yago:museum rdfs:label +dbpedia-owl:abstract +dbpprop:website +foaf:depiction +dbpprop:latitude +dbpprop:longitude +dbpedia-owl:location yago:location rdfs:label +dbpedia-owl:abstract +geo:lat +geo:long +dbpedia-owl:birthplace, deathplace, nationality dbpedia-owl:person +rdfs:label +dbpedia-owl:birthdate +dbpedia-owl:deathdate +dbpedia-owl:abstract +foaf:depiction yago:artisticmovement rdfs:label +dbpedia-owl:abstract +dbpedia-owl:movement Art +dbpprop:museum yago:art rdfs:label +dbpprop:year +dbpedia-owl:abstract +foaf:depiction +dbpprop:artist rdfs:subclassof rdfs:subclassof yago:painting yago:sculpture