The Semantic Web: Ontologies and owl

• The Semantic Web:

• Ontologies and OWL

Introduction to the Semantic Web

History of the Semantic Web

• Web was “invented” by Tim Berners-Lee (amongst others), a physicist working at CERN

• TBL’s original vision of the Web was much more ambitious than the reality of the existing (syntactic) Web:

• TBL (and others) have since been working towards realising this vision, which has become known as the Semantic Web

• E.g., article in May 2001 issue of Scientific American…

Beware of the Hype

• Hype seems to suggest that Semantic Web means: “semantics + web = AI”

• “A new form of Web content that is meaningful to computers will unleash a revolution of new abilities”

• More realistic to think of it as meaning: “semantics + web + AI = more useful web”

• Realising the complete “vision” is too hard for now (probably)
• But we can make a start by adding semantic annotation to web resources

Where we are Today: the Syntactic Web

The Syntactic Web is…

• A hypermedia, a digital library

• A library of documents called (web pages) interconnected by a hypermedia of links

• A database, an application platform

• A common portal to applications accessible through web pages, and presenting their results as web pages

• A platform for multimedia

• BBC Radio 4 anywhere in the world! Terminator 3 trailers!

• A naming scheme

• Unique identity for those documents

• A place where computers do the presentation (easy) and people do the linking and interpreting (hard).

• Why not get computers to do more of the hard work?

Hard Work using the Syntactic Web…

Impossible (?) using the Syntactic Web…

• Complex queries involving background knowledge

• Find information about “animals that use sonar but are not either bats or dolphins”

• Locating information in data repositories

• Travel enquiries
• Prices of goods and services
• Results of human genome experiments

• 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

What is the Problem?

What information can we see…

What information can a machine see…

Solution: XML markup with “meaningful” tags?

But What About…

Machine sees…

Need to Add “Semantics”

• External agreement on meaning of annotations

• E.g., Dublin Core
• Agree on the meaning of a set of annotation tags
• Problems with this approach
• Inflexible
• 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

Ontology: Origins and History

• a philosophical discipline—a branch of philosophy that
• deals with the nature and the organisation of reality

• Science of Being (Aristotle, Metaphysics, IV, 1)

• Tries to answer the questions:

• What characterizes being?
• Eventually, what is being?

Ontology in Linguistics

Ontology in Computer Science

• An ontology is an engineering artifact:

• It is 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]

Structure of an Ontology

• Ontologies typically have two distinct 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
• Adult_Elephant is a concept whose members are exactly those elephants whose age is greater than 20 years

• 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

A Semantic Web — First Steps

• Extend existing rendering markup with semantic markup

• Metadata annotations that describe content/funtion of web accessible resources

• Use Ontologies to provide vocabulary for annotations

• “Formal specification” is accessible to machines

• A prerequisite is a standard web ontology language

• Need to agree common syntax before we can share semantics
• Syntactic web based on standards such as HTTP and HTML

Ontology Design and Deployment

• Given key role of ontologies in the Semantic Web, it will be essential to provide tools and services to help users:

• Design and maintain high quality ontologies, e.g.:
• Meaningful — all named classes can have instances
• Correct — captured intuitions of domain experts
• Minimally redundant — no unintended synonyms
• Richly axiomatised — (sufficiently) detailed descriptions
• 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

Example Ontology