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Semantic Web Ontologies. CS 431 – 20040317 Carl Lagoze – Cornell University. Acknowledgements: Alun Preece. Components of the Semantic Web. Knowledge Representation and AI meeting the Web. Knowledge representation is not new Formalisms Semantic networks Frame systems Frames (objects)

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semantic web ontologies

Semantic Web Ontologies

CS 431 – 20040317

Carl Lagoze – Cornell University

Acknowledgements: Alun Preece

knowledge representation and ai meeting the web
Knowledge Representation and AI meeting the Web
  • Knowledge representation is not new
    • Formalisms
      • Semantic networks
      • Frame systems
        • Frames (objects)
        • Slots (attributes and properties)
    • Languages
      • LOOM
      • Classic
      • Cyc-L
    • Logics
      • Description Logics (DLs)
so why re invent it for the web
So why re-invent it for the Web
  • Not re-invention
    • Same underlying formalisms (frames, slots, description logic)
  • But new factors
    • Massive scale
      • Tractability
      • Knowledge expressiveness must be limited or reasoning must be incomplete
    • Lack of central control
      • Need for federation
      • Inconsistency, lies, re-interpretations, duplications
      • New facts appear and modify constantly
    • Open world vs. Close world assumptions
      • Contrast to most reasoning systems that assume anything absent from knowledge base is not true
      • Need to maintain monotonicity with tolerance for contradictions
    • Need to build on existing standards
      • URI, XML, RDF
why aren t xml schema good enough
Why aren’t XML schema good enough?
  • XML schema is designed to primarily describe structure and constraints on documents
    • Rich datatypes
    • Grammar for describing structure of elements (and attributes)
  • Incomplete modeling of is-a relationships
    • Top-down inheritance of attributes from superclasses to subclasses with extension and restriction is clumsy
      • E.g., student as person with a student # and age<28
    • Bottom-up inheritance of instances to superclasses is not automatic
brief review of rdf
Brief review of RDF
  • Data model for defining machine-processible semantics of data
  • Three main object types:
    • Resources
      • Primitive metadata is URI
    • Properties
      • Sub-class of resource
    • Statements
      • (:s :p :o)
  • Graph model that is serializable in XML
rdf schemas a building block for assembling ontology languages
RDF Schemas A building block for assembling ontology languages
  • Declaration of vocabularies
    • Classes and class hierarchies
    • properties defined by a particular community
    • characteristics of properties and/or constraints on corresponding values
  • Provides substructure for inferences based on existing triples
  • Schema language is an expression of basic RDF model
  • Schema Type System - Basic Types
    • Property, Class, SubClassOf, Domain, Range
    • Minimal (but extensible) at this time
    • Expressible in the RDF model and syntax
what is an ontology
What is an Ontology?
  • A formal specification of conceptualization shared in a community
  • Vocabulary for defining a set of things that exist in a world view
  • Formalization allows communication across application systems and extension
  • Parallel concepts in other areas:
    • Domains: database theory
    • Types: AI
    • Classes: OO systems
    • Types/Sorts: Logic
  • Global vs. Domain-specific
slide9

“An ontology is an explicit, partial specification of a conceptualization that is expressible as a meta-level viewpoint on a set of possible domain theories for the purpose of modular design, redesign and reuse of knowledge-intensive system components”

(Schreiber, Wielinga & Janswijer, 1995)

Relevant relations

Universe of discourse

<{a, b, c, d, e}, {on, above, clear, table}>

xml and rdf are ontologically neutral
XML and RDF are ontologically neutral
  • No standard vocabulary just primitives
    • Resource, Class, Property, Statement, etc.
  • Compare to classic first order logic
    • Conjunction, disjunction, implication, existential, universal quantifier
components of an ontology
Components of an Ontology
  • Vocabulary (concepts)
  • Structure (attributes of concepts and hierarchy)
  • Logical characteristics of concepts & attributes
    • Domain and range restrictions
    • Properties of relations (symmetry, transitivity)
wordnet
Wordnet
  • On-line lexical reference system, domain-independent
  • >100,000 word meanings organized in a taxonomy with semantic relationships
    • Synonymy, meronymy, hyponymy, hypernymy
  • Useful for text retrieval, etc.
  • http://www.cogsci.princeton.edu/~wn/online/
slide13
CYC
  • Effort in AI community to accommodate all of human knowledge!!!
  • Formalizes concepts with logical axioms specifying constraints on objects and classes
  • Associated reasoning tools
  • Contents are proprietary but there is OpenCyc
    • http://www.opencyc.org/
ontologies for the web
Ontologies for the Web
  • Lots of Participants and $$$
    • Web Ontology Working Group
    • Distributed Agent Markup Language
    • Ontology Inference Layer
    • OntoWeb
    • Schemas Project
  • OWL (Web Ontology Language) – develop standard for encoding ontologies on top of RDF Schema
web ontology language owl
Web Ontology Language (OWL)
  • W3C Web Ontology Working Group (WebOnt)
  • Follow on to DAML, OIL efforts
  • W3C Recommendation
  • Vocabulary extension of RDF
extending rdf s with owl
Extending RDF(S) with OWL

RDFS

OWL

  • Class definition: Conjunction, disjunction, negation
  • Property constraints: universality, existence, cardinality
  • Properties of properties: transitivity, symmetry
  • Class, sub-class definition
  • Property (attribute), sub-property definition
  • Domain, range constraints
species of owl
Species of OWL
  • OWL Lite
    • Good for classification hierarchies with simple constraints (e.g., thesauri)
    • Reasoning is computational simple and efficient
  • OWL DL
    • Computationally complete and decidable (computation in finite time)
    • Correspondence to description logics (decidable fragment of first-order logic)
  • OWL Full
    • Maximum expressiveness
    • No computational guarantees (probably never will be)
  • Each language is extension of simpler predecessor
description logics
Description Logics
  • Fragment of first-order logic designed for logical representation of object-oriented formalisms
    • Frames/classes
    • Do not stress representation of behaviors
  • High expressivity with decidability and completeness
  • Universal and existential quantification, disjunction, conjunction, negation
  • Concepts, roles, individuals
  • Representation as a collection of statements, with unary and binary predicates that stand for concepts and roles, from which deductions can be made
why owl class vs rdfs class
Why owl:class vs. rdfs:class
  • Rdfs:class is “class of all classes”
  • In DL class can not be treated as individuals (undecidable)
  • Thus owl:class, which is expressed as rdfs:subclass of rdfs:class
    • No problem for standard rdf processors since an owl:class “is a” rdfs:class
  • Note: there are other times you want to treat class of individuals
    • Class drinkable liquids has instances wine, beer, ….
    • Class wine has instances merlot, chardonnay, zinfandel, …
owl class building operations
OWL class building operations
  • disjointWith
    • No vegetarians are carnivores
  • sameClassAs (equivalence)
  • Enumerations (on instances)
    • The Ivy League is Cornell, Harvard, Yale, ….
  • Boolean set semantics (on classes)
    • Union (logical disjunction)
      • Class parent is union of mother, father
    • Intersection (logical conjunction of class with properties)
      • Class WhiteWine is conjunction of things of class wine and have property white
    • complimentOf (logical negation)
      • Class vegetarian is disjunct of class carnivore
owl property building operations restrictions
OWL property building operations & restrictions
  • Transitive Property
    • P(x,y) and P(y,z) -> P(x,z)
  • SymmetricProperty
    • P(x,y) iff P(y,x)
  • Functional Property
    • P(x,y) and P(x,z) -> y=z
  • inverseOf
    • P1(x,y) iff P2(y,x)
  • InverseFunctional Property
    • P(y,x) and P(z,x) -> y=z
  • Cardinality
    • Only 0 or 1 in lite and full
owl datatypes
OWL DataTypes
  • Full use of XML schema data type definitions
  • Examples
    • Define a type age that must be a non-negative integer
    • Define a type clothing size that is an enumeration “small” “medium” “large”
owl instance creation
OWL Instance Creation
  • Create individual objects filling in slot/attribute/property definitions

<Person ref:ID=“William Arms”> <rdfs:label>Bill</rdfs:label> <age><xsd:integer rdf:value=“57”/></age> <shoesize><xsd:decimal rdf:value=“10.5”/></shoesize></Person>