Using Rules with Ontologies in the Semantic Web

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Using Rules with Ontologies in the Semantic Web

• Chimezie OgbujiThoracic and Cardiovascular
  SurgeryCleveland Clinic Foundation

July 25th, 2006 Presented to W3C HCLSIG ACPP Group
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Rule Languages

• RuleML
• SWRL
• Notation 3 (N3)

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Semantic Web Reasoners

• Closed World Machine (CWM)
• Euler
• Pychinko
• RDFEngine
• Jena

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Using Rules to Describe Rules

• A Rule consists of a body (antecedent) and a head
  (consequent)
• In N3
• ?X has ?body. ?X has ?head. body logimplies
  head gt ?X a Rule
• In SWRLs abstract syntax
• Implies(Antecedent(has(I-variable(X),
  I-variable(body) has(I-variable(X),
  I-variable(head)) logImplies(I-variable(body),
  I-variable(head))) Consequent(Rule(I-variable(X)))

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Origin / Background of Rules

• Logic Programming
• Production systems (RETE, etc.)
• Prolog
• SQL
• Horn-clause Logic
• Efficient theorem proving, and reasoning

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Complexity and Abstraction

• Kurt Cagle
• You can never eliminate complexity from a
  system, you can only move it from place to place
• Important point regarding how / why rules are
  used with ontology languages

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DL Semantics as Analogy for Rules

• Description Logics (DL)
• Strict subset of FOL with decidability in mind
• DL are more palatable and (therefore) more
  ubiquitous
• Ontology language constructs correspond to DL
  constructs
• Some DL (ontology) reasoning can be done via
  explicit rules

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Some DL Semantics as N3 Rules

• Transitive Roles
• ?P a owlTransitiveProperty. ?X ?P ?Y. ?Y ?P ?Z
  gt ?X ?P ?Z
• Class inclusion
• ?B rdfssubClassOf ?C. ?A rdfssubClassOf ?B gt
  ?A rdfssubClassOf ?C.
• Inverse Roles
• ?P owlinverseOf ?Q. ?S ?P ?O gt ?O ?Q ?S.
• Functional Restrictions
• ?P a owlFunctionalProperty. ?S ?P ?X. ?S ?P ?Y
  gt ?X owlsameAs ?Y.

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Restriction on Expressivenes

• Certain implications cannot be expressed in DL
• Individuals who live and work at the same
  location are Home Workers
• In N3
• ?X work ?Y. ?X live ?Z. ?Z located ?W. ?Y
  located ?Y gt ?X a HomeWorker

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Restriction on Expressiveness (Cont.)

• Specific logical restrictions (Horn logic) make
  it difficult to express certain statements
• Every person has a father (known or unknown)
• However, this is straight forward in OWL (and
  Description Logics)
• Person a owlClass
• rdfssubClassOf
• a owlRestriction
• owlonProperty father
• owlcardinality 1.

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Rule Inference Methods

• Given
• Set of rules
• Set of facts
• Backward chaining is goal-oriented
• Question can a fact be inferred from the rules
  and existing facts?
• Forward chaining exhaustively infers new facts
  from the rules
• The resulting facts combined with the original
  facts are often referred to as the closure

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Concerns with Reasoning

• Logic Programming reasoners and algorithms are
  more mature
• RETE algorithm for production (forward chaining)
  systems
• Euler cycle detection for backward chaining
  inference
• Logic Programming systems are at the mercy of the
  explicit rules

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Complexity and Abstraction (Revisited)

• Kurt Cagle
• You can never eliminate complexity from a
  system, you can only move it from place to place
• DL abstracts Knowledge Representation at the
  expense of the reasoning mechanism.
• DL reasoners are implemented to support only a
  limited kind of inference class subsumption and
  consistency detection.

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Compromise

• Use DL semantics where the domain falls nicely
  into Categories / Roles and decidability is an
  issue
• Use rules everywhere else
• The combination covers the full spectrum of
  expressiveness and decidability
• Including both in the thought process improves
  Knowledge Engineering
• Some DL semantics can be expressed as rules to
  take advantage of efficient pattern matching
  algorithms

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References

• Description Logic Programs Combining Logic
  Programs with Description Logic
• http//citeseer.ist.psu.edu/grosof03description.ht
  ml
• Description Logic Complexity Navigator
• http//www.cs.man.ac.uk/ezolin/logic/complexity.h
  tml
• Web Ontology Reasoning with Logic Databases
• http//www.ubka.uni-karlsruhe.de/vvv/2004/wiwi/2/2
  .pdf
• Eulers RDF Plus OWL N3 Rules
• http//www.agfa.com/w3c/euler/rpo-rules.n3
• Description Logics as Ontology Languages for the
  Semantic Web
• http//citeseer.ist.psu.edu/baader03description.ht
  ml