Ontology & Semantic Web – A Dummy’s Overview of Modern

Technologies for Sharing KnowledgeMitsunori Ogihara

Center for Computational Science

What Is an Ontology?

• Merriam-Webster: “The branch of metaphysics dealing with the nature of being”– What does it mean to exist?– What exists?

• In the field of computer science an ontology is “a specification of a conceptualization” – Tom Gruber

World, Specification, Conceptualization

• Human observes the world and conceptualizes it

• That human conceptualization is put into a specification

• The world matches the specification

What an Ontology Can Conceptualize

• Things to exist– Individuals, not necessarily physical existence– Classes of individuals

• Relations among things– Is a part of– Is not equal to

• Properties about things– Has a value of

Problem

• Conceptualization is ambiguous and inaccurate– How a person A sees the world is not necessarily

equal to how a person B sees the world• Specification is difficult– Formal specification is tiresome

• How efficiently can one develop an ontology?• How efficiently can one compare ontologies?

Why Was the Idea of Ontology Created?

• Artificial Intelligence … a branch of computer science that studies computational methods of mimicking human intelligence

• Intelligence includes ability to– Understand data obtained through senses– Acquire knowledge– Apply knowledge to solve problems– Understand emotion

Knowledge Representation

• An area that studies how to formally think– [Davis, Shrobe, and Solovitz’93] Knowledge

Representation is• A surrogate• A set of ontological commitments• A fragmentary theory of intelligent reasoning• A medium for efficient computation• A medium of human expression

– Commitments are filters through which the world is observed

Semantic Web

• The first generation of Web is HTML (Hypertext Markup Language)– This is designed so as to present texts in a format

specification that can be easily understood and rendered

– Search engines can find documents that may contain certain information by using keyword matches, but can’t find an answer to a question

Semantic Web• A new generation of web should provide not texts but

structured information, a part of which may be texts– Resource Description Framework (where the resources

are)– XML (Extensive Markup Language)

• A user-definable format• Documents conforming to the format

• Idea:– Decide on what information can a web page might contain– Decide on how to describe such information– Annotate the web page with such information in a

predetermined format

Ontology Development Tools

• OWL (Web Ontology Language)– Currently the most popular ontology description

language– http://www.w3.org/TR/owl-features/– OWL DL (Description Logic, standard version)– OWL Lite (restricted version) … basic constructs

exist to logically express constructs of DL– OWL Full (for RDF)– http://www.cs.manchester.ac.uk/~horrocks/ISWC

2003/Tutorial/examples.pdf

A History of Ontology Description Languages

• KIF (1992) … Stanford, first-order logic• Loom (1992) … USC, first-order logic, for KR

nor necessarily for ontologies• FLogic (1995) … Karlsruhe, combination of

first-order logic and frames• OKBC (1997) … DARPA• XOL (1999) … SRI, an XML version of OKBC• OWL (2001) … W3C

Ontology Development Tools

• Created along with development of description languages

Popular Free Tools

• Protégé-2000• Swoop … an open source project, hosted at

Google

Ontology Building Process• Vocabulary– Need to settle on a set of words to be used to describe the

domain knowledge (or the domain of the web contents)– Where to start? Thousands of words?

• Knowledge Base Building– Express domain experts’ knowledge in terms of ontology– Who will translate knowledge into logical forms?

Ambiguity issues?• Inference– Make new discovery– Identify classes and properties of an individual– Inference engines, compute-intensive

Exporting Ontologies

• Protégé and Swoop (and others) have the ability to export/import data in various formats– Enables information exchange between ontologies

Finding a Nice Mapping• A mapping f of an ontology O to an ontology O’ is one that

maps each class of O to a class of O’ and each property of O to another property of O’. We want:– For all classes c and d of O, c is a subclass of d if and only if f(c) is

a subclass of f(d) in O’– For all class c and property p of O, c has property p if and only if

f(c) has property f(p) in O’• Finding a perfect mapping is hard, and practically such a

perfect mapping rarely exists• Finding a mapping that maximizes a certain quantity is also

difficult, and is NP-hard– Heuristic methods are usually used (based on graph properties)

References• T.R.Gruber (1993), A Translation Approach to Portable Ontology

Specifications, Knowledge Acquisition• V.Devedzik(2002), Understanding ontological engineering,

Communications of the ACM• J.Gennari, M.Musen, R.Fergerson (2003), The evolution of Protégé:

an environment for knowledge-based systems development, International Journal of Human-Computer Studies

• A.Kalyanpur, B.Parsia, E.Sirin, B.Grau (2006), Swoop: A web ontology editing browser, Web Semantics: Science

• O.Corcho et al. (2003), Methodologies, tools and languages for building ontologies. Where is their meeting point? Data&Knowledge Engineering

• L.Lacy (2005), OWL: Representing information using the web ontology language

• J.Euzenat, P. Shvaiko (2007), Ontology Matching, Springer