DAML+OIL is not Enough

Sean Bechhofer, Ian Horrocks, Carole GobleInformation Management Group

University of Manchester, UKhttp://img.cs.man.ac.uk

Semantic Web Working Symposium

Stanford, USAJuly 30 - August 1, 2001

Take Home Message

• OIL was good and popular.• DAML+OIL is still good, but….• The move away from a frame-like representation may

make modelling harder. • Even more need for good tool support.

OIL: The Three Roots

• Frame-based Representations;• Description Logics;• Web Languages.

Ontologies & Lexicons

Frame-based Systems:Epistemological ModellingPrimitives

Web Languages:XML- and RDF-basedsyntax

Description Logics:Formal Semantics & Reasoning Support

OIL

OIL: Frames

• Modelling primitives based on classes (frames) with local attributes.

• Provide a natural and friendly representation for modellers. Has proved popular with, e.g. bioinformaticians.

Sometimes a lack of well-defined semantics.UK-Animal-lover

Superclass Person

Has-pet > 3 Animal

Lives-in UK

UK-Animal-Lover Person .livesin UK atleast 3 has-pet Animal

OIL: Description Logics

• Describe knowledge in terms of concepts and roles. • Generally supply a range of concept forming operators:

– boolean connectives and, or, not and role restrictions

• Well-defined, formal semantics allowing the use of reasoners.

Hard(er) to interact with directly.

<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns:dc="http://purl.org/dc/elements/1.1/" xmlns:oil="http://www.ontoknowledge.org/oil/rdf-schema/2000/11/10-oil-standard#" xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#"> 

<rdf:Description rdf:about="" dc:creator="Sean Bechhofer" dc:description="An Ontology about animals, built with OilEd" dc:description.release="1.0" dc:identifier="id" dc:language="OIL" dc:title="Animals" dc:type="ontology" />

<rdfs:Class rdf:about=“http://cohse.semanticweb.org/ontologies/elephants.rdfs#elephant">  <rdfs:comment>Big grey animals with a trunk and large ears. Belonging to the family of pachyderms, elephants can be subdivided into the class of Indian elephants, i.e. those coming from India, and African elephants, those coming from Africa. One of them has bigger ears than the other, but I can never remember which one it is.</rdfs:comment>   <oil:hasPropertyRestriction rdf:resource="_anon12" />   <oil:hasPropertyRestriction rdf:resource="_anon13" />   <rdfs:subClassOf rdf:resource="file:F:/Projects/OIL/OilEd/ontologies/elephants.rdfs#animal" />  

</rdfs:Class></rdf:RDF>

OIL: Web languages

• A representation language needs a delivery format and concrete syntax.

• OIL originally specified using XML DTD, later with RDF Schema.

• Ensures compatability with existing tools and gives RDFS-aware applications access to OIL ontologies.

RDF isn’t really human-readable, thus tools are needed.

DAML+OIL

• Bringing together DAML’s ontology language and OIL.• Keeps OIL’s mapping to Description Logic. • Keeps the Web language mapping.• Moves away from frame-style language.

– General axioms rather than frame descriptions.– Essentially an alternative syntax for a DL.

• OilEd: a simple ontology editor.• Originally developed to demonstrate the use of a

reasoner with OIL. • Exploits the frame-based

paradigm of OIL. But also extends pure frames, allowing arbitrary expressions.

• Download from: http://img.cs.man.ac.uk/oil

• Lunchtime demo tomorrow!

DAML+OIL vs. OIL

• The semantics is still preserved, but.. It becomes harder to reconcile frame-based

descriptions with their underlying representation in DAML+OIL.

Information about how the descriptions have been structured is lost.

DAML+OIL applications

What does it mean for an application to use DAML+OIL?1. writing out a model as DAML+OIL;2. reading it back in from the DAML+OIL;3. reading someone else’s DAML+OIL.

White Van Man: frames

class-def defined white_van_man subclass-of man slot-constraint drives has-value (van and

(slot-constraint has_colour has-value (one-of white)))

implies white_van_man aggressive_driver

Basic Definition

Axioms

White Van Man: axioms I

equivalent white_van_man (man and slot_constraint drives has-value (van and (slot-constraint has_colour has-value (one-of white)))

implies white_van_man aggressive_driverAxioms

• How do I know which axiom corresponds to the definition?

White Van Man: axioms IIimplies white_van_man man

implies white_van_man slot_constraint drives has-value (van and (slot-constraint has_colour has-value (one-of white)))

implies (man and slot_constraint drives has-value (van and (slot-constraint has_colour has-value (one-of white))) white_van_man

implies white_van_man aggressive_driver

Axioms

Is this a problem?

• Not for delivery and use of ontologies. • Modelling and exchange require a common understanding of the

model. • If we adopt the frame-based paradigm for modelling, we may not

be able to consistently recreate the original model from the axioms.

• Tools may have to guess the modeller’s intended structure.• Good understanding cannot be ensured by meaning preservation

[Euzenat]

Solutions?

• Extend the schema locally (i.e. tool-specific information) to represent the structure of the information.

• Fine if everyone uses the same tool. Moves away from the standard, and introduces bespoke

representations that may not be easily exchanged.

Concluding Remarks

• Some of OIL’s good features may have been lost in DAML+OIL

• RDF syntax provides freedom of expression, but…– Provides no clear structure– May make modelling harder– Is notoriously baroque

• Requirement for tools, but…– Lack of structure has serious implications for tool builders