Semantic Web and its Logical Foundations

Serguei Krivov, Ecoinformatics Collaboratory

Gund Institute for Ecological Economics, UVM

Overview

• Semantic Web

• Semantic Web Languages and Tools

• Description Logics (DL) – Formalism of Web Ontology Languages

• OWL –latest Web Ontology Language

Why Semantic Web?

• Most information on the Web is designed for human consumption, and even if it was derived from a database with well defined meanings for its columns, that the structure of the data is not evident to a robot browsing the web.

• Leaving aside the artificial intelligence problem of training machines to behave like people, the Semantic Web approach instead develops languages for expressing information in a machine processable form.

The Semantic Web Vision

+ Web made possible through established standards TCP/IP for transporting bits down a wire HTTP & HTML for transporting and rendering hyperlinked text+ Applications able to exploit this common infrastructureResult is the WWW as we know it+ 1st generation web mostly handwritten HTML pages+ 2nd generation (current) web often machine generated/active+ Both intended for direct human processing/interaction+ In next generation web, resources should be more accessible to

automated processesTo be achieved via semantic markupMetadata annotations that describe content/function+ Coincides with Tim Berners-Lee’s vision of a Semantic Web

Ian Horroks, Logical Foundations for the Semantic Web

What is Semantic Web?• Semantic Web is an initiative of World Wide Web consortium (

http://www.w3.org/2001/sw/)

• Semantic Web is a set of Languages and Tools for machine processing of information stored in WWW

• You can think of it as being an efficient way of representing data on the World Wide Web, or as a globally linked Knowledge Base

. • Semantic Web is about efficient Knowledge Representation mechanism for

WWW. (AI)

• Semantic Web is about efficient Reasoning Systems requered for integration of distributed data .

Ontologies

+ Semantic markup must be meaningful to automated processes+ Ontologies will play a key role Source of precisely defined terms (vocabulary) Can be shared

across applications (and humans)

+ Ontology typically consists of: Hierarchical description of important concepts in domain Descriptions of properties of instances of each concept+ Degree of formality can be quite variable + Increased formality and regularity facilitates machine understanding

+ Ontologies can be used, e.g.: To facilitate agent-agent communication in e-commerce In semantic based search To provide richer service descriptions that can be more flexibly

interpreted by intelligent agents

Ian Horroks, Logical Foundations for the Semantic Web

RDF –Ressource Definition Framework

<?xml version="1.0"?><Class rdf:ID="Resource" xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns="uri"> <property>value</property> <property>value</property> ...</Class>

[ressource] [property] [ressource]

[Hamlet] [Author] [W. Shakespeare ]

RDF Usage– Add ”annotations” for web-pages ?– Add ”annotations” for web-services ?– Instead of building a menu for a web-site,

declare the content in RDF, and let the user-agent build the menu dynamically ?

– Describe all images on your website ?– Market the web.service your web.site provides

?

RDF is used as a syntax for more expressive languages: DAML+OIL, OWL

*.jpgRDF enhanced W

ebsite

”Semantic Web In A Box”

• A Semantic Web in A Box Architecture

RDF Repository

Search View Update ManageInternetInternet

Agents

*.gif *.asp*.html

*.jsp

From Jens Jacob Anderson’s Semantic Web Tutorial

Companies ,Tools, and Resources

Ontoprise : OntoEdit, OntoBrocker see: http://www.ontoprise.de/products/index_html_en

Unicorn http://www.unicorn.com/

KAON : http://kaon.semanticweb.org/

Protégé -Free Ontology Editor http://protege.stanford.edu/

Semantic Web Tools Tutorialhttp://www.daml.org/2003/05/swmu-tools-tutorial/Overview.html

First Semantic Web Website: http://owl.mindswap.org/

Web Languages

+ Web languages already extended to facilitate content descriptionXML Schema (XMLS)RDF and RDF Schema (RDFS)

+ RDFS recognisable as an ontology languageClasses and propertiesRange and domain of propertiesSub/super-classes (and properties)

+ But RDFS not a suitable foundation for Semantic WebToo weak to describe resources in sufficient detail

+ Requirements for web ontology language:Compatible with existing Web standards (XML, RDF,

RDFS) Easy to understand and use (based on familiar KR idioms) Formally specified and of “adequate” expressive power possible to provide automated reasoning support

Ian Horroks, Logical Foundations for the Semantic Web

Formalism for Ontology Languages: Description Logicsextracts from Franconi’s course

DL-examples

DL definition of “Happy Father”

(Example from Ian Horrocks, U Manchester, UK)

A Few Fundamental Definitions

Example:

KB={

A&B,

(C or (not D)),

D

}The following is true:

KB |= A

KB |= C

• Decidability: A formal system is decidable if it has inference procedure i that always terminates with definite answer (either KB|-A or not KB|-A)

Why DL are used as formalism of Semantic Web ?

• FOL –sound, complete, but not decidable

• Proposition Logic- sound, complete, decidable, but has low expressive power

• DL- has sound, complete, decidable inference procedure; has reasonable expressive power

• Alternatives to DL- decidable fragments of FOL- Horn logic (prolog), F-Logic

OWL-Web Ontology LanguageExtracts from: Ian Horroks, Logical Foundations for the Semantic Web

+ Three species of OWLOWL full is union of OWL syntax and RDFOWL DL restricted to FOL fragment ( DAML+OIL)

OWL Lite is “easier to implement” subset of OWL DL

+ Semantic layeringOWL DL OWL full within DL fragmentDL semantics officially definitive

+ OWL DL based on SHIQ Description Logic

+ Benefits from many years of DL researchWell defined semanticsFormal properties well understood (complexity,

decidability) Known reasoning algorithmsImplemented systems (highly optimised)