TU Wien – April 24-29, 2006Semantics and Ontologies in GI Services Ontologies for the Semantic Web Michael Lutz Slides based on Co-ode OWL Tutorial (University of Manchester)

TU Wien – April 24-29, 2006Semantics and Ontologies in GI Services Overview Why do we need ontologies and the Semantic Web? What are ontologies  ontology definitions  properties Ontology languages  OWL & Description Logics  Reasoning

TU Wien – April 24-29, 2006Semantics and Ontologies in GI Services Impossible (?) using the Syntactic Web… Complex queries involving background knowledge  Find information about “animals that use sonar but are not either bats or dolphins”, e.g., Barn owl Locating information in data repositories  Travel enquiries  Prices of goods and services Finding and using web services  Visualise surface interactions between two proteins Delegating complex tasks to web agents  Book me a holiday next weekend somewhere warm, not too far away, and where they speak French or English

TU Wien – April 24-29, 2006Semantics and Ontologies in GI Services Consider a typical web page: Markup consists of: rendering information (e.g., font size and colour) Hyper-links to related content Semantic content is accessible to humans but not (easily) to computers… What is the Problem?

TU Wien – April 24-29, 2006Semantics and Ontologies in GI Services What information can we see… WWW2002 The eleventh international world wide web conference Sheraton waikiki hotel, Honolulu, hawaii, USA, 7-11 may location. 5 days. learn. interact Registered participants coming from: australia, canada, chile denmark, france, germany, … Register now On the 7th May Honolulu will provide the backdrop of the eleventh international world wide web conference. This prestigious event … Speakers confirmed  Tim Berners-Lee (Tim is the well known inventor of the Web, …) ……

TU Wien – April 24-29, 2006Semantics and Ontologies in GI Services                         What information can a machine see…

TU Wien – April 24-29, 2006Semantics and Ontologies in GI Services                        … XML markup with “meaningful” tags The Solution?

TU Wien – April 24-29, 2006Semantics and Ontologies in GI Services       …        , …           … Machine sees…

TU Wien – April 24-29, 2006Semantics and Ontologies in GI Services Need to Add “Semantics” External agreement on meaning of annotations  e.g., Dublin Core: agree on the meaning of a set of annotation tags  but this approach is inflexible and only a limited number of things can be expressed Use ontologies to specify meaning of annotations  Ontologies provide a vocabulary of terms  New terms can be formed by combining existing ones  Meaning (semantics) of such terms is formally specified  Can also specify relationships between terms in multiple ontologies

TU Wien – April 24-29, 2006Semantics and Ontologies in GI Services Ontology in Computer Science An ontology is an engineering artifact:  constituted by a specific vocabulary used to describe a certain reality, plus  a set of explicit assumptions regarding the intended meaning of the vocabulary. Thus, an ontology describes a formal specification of a certain domain:  Shared understanding of a domain of interest  Formal and machine manipulable model of a domain of interest “An explicit specification of a conceptualisation” [Gruber93]

TU Wien – April 24-29, 2006Semantics and Ontologies in GI Services Ontology Components Names for important concepts in the domain  Elephant is a concept whose members are a kind of animal  Herbivore is a concept whose members are exactly those animals who eat only plants or parts of plants Background knowledge/constraints on the domain  Adult_Elephants weigh at least 2,000 kg  All Elephants are either African_Elephants or Indian_Elephants  No individual can be both a Herbivore and a Carnivore

TU Wien – April 24-29, 2006Semantics and Ontologies in GI Services A semantic continuum [Mike Uschold, Boeing Corp] Shared human consensus Implicit Text descriptions Pump: “a device for moving a gas or liquid from one place or container to another” Informal (explicit) Semantics hardwired; used at runtime Formal (for humans) Semantics processed and used at runtime (pump has (superclasses (…)) Formal (for machines) Less ambiguity Better interoperation More robust – less hardwiring More difficult  Further to the right 

TU Wien – April 24-29, 2006Semantics and Ontologies in GI Services Ontology Design and Deployment In the Semantic Web there should be tools and services to help users:  Design and maintain high quality ontologies  Store (large numbers) of instances of ontology classes, e.g. annotations from web pages  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

TU Wien – April 24-29, 2006Semantics and Ontologies in GI Services Clash of intuitions: Domain Experts vs. Logicians  Transparency & predictability vs. Rigour & Completeness  Neophytes caught in the muddled middle The knowledge acquisition “bottleneck” Assuring quality & managing change Confusion of terminology and usage Interdisciplinarity: Linguistics, Cognitive science, Software engineering, Philosophy A jumble of syntaxes Why Ontology Engineering is hard…

TU Wien – April 24-29, 2006Semantics and Ontologies in GI Services “Class”  “Concept”  “Category”  “Type”  are often used synonymously, but more precisely… -categories are in the world -concepts are in the mind -classes/types are in the ontology “Instance”  “Individual” “Entity”  “object”  can be class or individual “Property”  “Slot”  “Relation”  “Relationtype”  “Attribute”  Semantic link type”  “Role” Vocabulary

TU Wien – April 24-29, 2006Semantics and Ontologies in GI Services Ontologies Software agents Problem- solving methods Domain- independent applications Domain- independent applications Databases Declare structure Knowledge bases Knowledge bases Provide domain description The “Semantic Web” An Ontology should be just the Beginning

TU Wien – April 24-29, 2006Semantics and Ontologies in GI Services Ontology Languages Wide variety of languages for “Explicit Specification”  Graphical notations, e.g. UML, Semantic networks, Topic Maps (see RDF  Logic based, e.g. Description Logics (e.g., OWL), Rules (e.g., RuleML, LP/Prolog), First Order Logic (e.g., KIF), Conceptual graphs, higher order and non-classical logics  Probabilistic/fuzzy Degree of formality varies widely  Increased formality makes languages more amenable to machine processing (e.g., automated reasoning)

TU Wien – April 24-29, 2006Semantics and Ontologies in GI Services 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 Language Primitives

TU Wien – April 24-29, 2006Semantics and Ontologies in GI Services Web Ontology Language Requirements Desirable features for Web Ontology Language:  Extends existing Web standards, e.g. 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

TU Wien – April 24-29, 2006Semantics and Ontologies in GI Services OWL Language Three species of OWL  OWL full is union of OWL syntax and RDF  OWL DL restricted to FOL fragment (  DAML+OIL)  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)

TU Wien – April 24-29, 2006Semantics and Ontologies in GI Services (In)famous “Layer Cake”  Data Exchange  Semantics+reasoning  Relational Data ? ? ??? TimBL, 2000 ( Talks/1206-xml2k-tbl/slide10-0.html)

TU Wien – April 24-29, 2006Semantics and Ontologies in GI Services References Daconta, M.C., Obrst, L.J. & Smith, K.T. (2003): The Semantic Web. A Guide to the Future of XML, Web Services, and Knowledge Management, John Wiley & Sons  esp. chapter 8: Understanding Ontologies McGuinness, D.L. & van Harmelen, F. (Eds.) (2004): OWL Web Ontology Language Overview, available from Baader, F., Calvanese, D., McGuinness, D., Nardi, D. & Patel- Schneider, P. (2003): The Description Logic Handbook, Cambridge University Press  esp. introductory chapters 1 & 2

TU Wien – April 24-29, 2006Semantics and Ontologies in GI Services Questions???