Ontology & Semantic Web – A Dummy's Overview of Modern ...

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Ontology & Semantic Web


A
Dummy’s Overview of Modern
Technologies for Sharing Knowledge

Mitsunori 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
conceptualizatio
n 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