An Introduction to Ontologies

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An Introduction to Ontologies

• Tim Finin
• University of Maryland Baltimore County

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What is an ontology

• The subject of ontologyis the study of
  thecategories of things thatexist or mayexist
  in some domain.
• The word ontology isfrom the Greek ontos
  forbeing and logos for word.
• Aristotle offered an ontology which included 10
  categories, shown as the leaves in this tree
  (from Sowa, after Brentano)

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Tree ofPorphyry

• The oldest knowntree diagram is the3rd century
  AD work by Greek philosopherPorphyry in
  commentary on Aristotle.
• Substance was identified as the supreme genus or
  the most general supertype.

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Top down vs. bottom up

• Philosophers build fromthe top down and
  areinterested in capturingthe most
  generalconcepts.
• Programmers tend towork from the bottomup,
  supporting a set ofapplications, with a little
  generality to help reuse and future development.
• Ex CHAT-80 system (Periera and Warren, 1982)
  which answered NL questions about a geographic
  database.
• Example of a microworld ontology supported NLP,
  query answering, and generation

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Blocks world
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Blocks world

• The blocks world is another microworld used
  often for NLP, vision, planning.
• It consists of a table, a set of blocks or
  different shapes, sizes and colors and a robot
  hand.
• Some typical domain constraints
• Only one block can be on another block.
• Any number of blocks can be on the table.
• The hand can only hold one block.
• Typical representation
• ontable(a) ontable(c)
• on(b,a) handempty
• clear(b clear(c)

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Trees, Lattices, and Other Hierarchies

• Most systems for expressing ontologies make heavy
  use of familiar representation schemes, including
  trees, lattices, acyclic graphs and general
  graphs
• A lattice has a TOP (everthing) and BOTTOM
  (nothing)

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Ontologies in Computer Science

• Ontology A common vocabulary and agreed upon
  meanings to describe a subject domain.

Ontol"ogy (?), n. Gr. the things which exist
(pl.neut. of , , being, p.pr. of to be) -logy
cf.F. ontologie. That department of the science
of metaphysics which investigates and explains
the nature and essential properties and relations
of all beings, as such, or the principles and
causes of being. Webster's Revised Unabridged
Dictionary (G C. Merriam Co., 1913, edited by
Noah Porter)

• This is not a profoundly new idea
• Vocabulary specification
• Domain theory
• Conceptual schema (for a data base)
• Class-subclass taxonomy
• Object schema

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Importance of ontologies in communication

• An example of the importance of ontologies in
  communication is the fate of NASAs Mars Climate
  Orbiter
• It crashed into Mars on September 23, 1999
• JPL used metric units in their program
  controlling the thrusters and Lockheed-Martin
  used imperial units.
• Instead of establishing an orbit at an altitude
  of 140km, it did so at 60km, causing it to burn
  up in the Martian atmosphere.

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Conceptual Schemas

• A conceptual schema specifies the intended
  meaning of concepts used in a data base

Data Base
Table price stockNo integer cost float
Data Base Schema
Auto Product Ontology
price(x, y) ? (x, y) auto_part(x)
part_no(x) x
retail_price(x, y, Value-Inc)
magnitude(y, US_dollars) y
Product Ontology
Conceptual Schema
Units Measures Ontology
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Implicit vs. Explicit Ontologies

• Systems which communicate and work together must
  share an ontology.
• The shared ontology can be implicit or explicit.
• Implicit ontology are typically represented only
  by procedures
• Explicit ontologies are (ideally) given a
  declarative representation in a well defined
  knowledge representation language.

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Conceptualizations, Vocabularies and
Axiomitization

• Three important aspects to explicit ontologies
• Conceptualization involves the underlying model
  of the domain in terms of objects, attributes and
  relations.
• Vocabulary involves assigning symbols or terms to
  refer to those objects, attributes and relations.
• Axiomitization involves encoding rules and
  constraints which capture significant aspects of
  the domain model.
• Two ontologies may
• be based on different conceptualizations
• be based on the same conceptualization but use
  different vocabularies
• differ in how much they attempt to axiomitize the
  ontologies

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Simple examples
fruit
tropical
temperate
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Ontologies vs. KBs

• Ontologies are distinguished from KBs not by
  their form, but by the role they play in
  representing knowledge
• Consensus models for a domain
• Emphasis on properties that hold in all
  situations
• Emphasis on classes rather than instances
• Intended to support multiple tasks and methods
• Dont change during problem solving and are
  suited for compiling into tools
• Need to satisfy a community of use
• Emphasis on collaborative development
• Emphasis on translation to multiple logical
  formalisms
• Useful for education

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Ontology Library and Editing Tools

• Ontolingua is a language for building,
  publishing, and sharing ontologies.
• A web-based interface to a browser/editor server
  at http//ontolingua.stanford.edu/ and mirror
  sites.
• Ontologies can betranslated into a number of
  content languages, including KIF, LOOM, Prolog,
  CLIPS, etc.
• Chimera is a tool for merging existing ontologies

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Big Ontologies

• There are several large, general ontologies that
  are freely available.
• Some examples are
• Cyc - Original general purpose ontology
• WordNet - a large, on-line lexical reference
  system
• World Fact Book -- 5Meg of KIF sentences!
• UMLS - NLMs Unified Medical Language System
• See http//www.cs.utexas.edu/users/mfkb/related.ht
  ml for more

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WordNet

• WordNet is an on-line lexical referencesystem
  whose design is inspired bypsycholinguistic
  theories of human lexicalmemory.
• English nouns, verbs, adjectives and adverbs are
  organized into synonym sets, each representing
  one underlying lexical concept.
• Synsets board,plankboard,committee
• Different relations link the synonym sets (e.g.
  antonyms, generalizations, etc)
• 140K words
• Developed by the Cognitive Science Laboratory at
  Princeton and available online
• Although linguistically motivated, many groups
  have used it as a general ontology of concepts.
• http//www.cogsci.princeton.edu/wn/

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EDR Electronic Dictionary

• http//www.iijnet.or.jp/edr/
• a dictionary with over 400,000 concepts, with
  their mappings to both English and Japanese
  words.

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Cyc

• CYC is a large KB which has beenunder continual
  development sinceabout 1985.
• The CYC KB is a formalized representation a vast
  quantity of fundamental human knowledge facts,
  rules of thumb, and heuristics for reasoning
  about the objects and events of everyday life.
• CYC is encoded in the KR language CYCL
• The Upper CYC Ontology contains approximately
  3,000 terms capturing the most general concepts
  of human consensus reality.
• http//www.cyc.com/cyc-2-1/cover.html

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Cycs top level concepts
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openCyc

• http//www.opencyc.org/
• OpenCyc 1.0 (summer 2002?) will include the
  following.
• 6,000 concepts an upper ontology for all of
  human consensus reality.
• 60,000 assertions about the 6,000 concepts,
  interrelating them, constraining them, in effect
  (partially) defining them.
• A compiled version of the Cyc Inference Engine
  and the Cyc Knowledge Base Browser.
• A specification of CycL, the language in which
  Cyc (and hence OpenCyc) is written. There are
  CycL-to-Lisp,CycL-to-C, etc. translators.
• A specification of the Cyc API
• A few sample programs that demonstrate use of the
  Cyc API for application development.

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IEEE Standard Upper Ontology

• An IEEE standards working group
• This standard will specify an upperontology
  that will enable computers to utilize it for
  applications such as data interoperability,
  information search and retrieval, automated
  inferencing, and natural language processing.
• http//suo.ieee.org/
• See site for documents and archives of mailing
  list discussions
• Two starter documents for SUOs SUMO, IFF

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World Fact Book

• Stanfords WFB aims to semi-automatically
  construct a substantial KB of basic geographic,
  economic, political, and demographic knowledge
  about the world's nations.
• Source CIA World Fact Book
• 5.2 MB 5K classes 64K facts and rules encoded
  in KIF
• Available from http//www-ksl-svc.stanford.edu591
  5/doc/wfb/ in several forms

• Example resources, industries, commodities
• Interrelated crude-oil reserves, production,
  exports
• Coal mining,computer industry,auto parts
  industry,
• Specify basic definitions
• A natural resource is a deposit of stuff an
  industry is a collection of businesses a
  commodity is an item whose sales can be measured
  as a continuous quantity
• Examine related classes identify key factors
• E.g., material, process, product, customer,
  location, task
• Define each industry as a conjunction of factors
• 6 generative factors discriminate 500 industries
• Organize values of factors (mining

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Unified Medical Language System

• Under development since 1986 by the National
  Library of Medicine
• Supports standardize medical terminology via a
  central dictionary thesaurus semantic
  network search engine
• Purpose is to aid the development of systems
  that help health professionals and researchers
  retrieve and integrate electronic biomedical
  information from a variety of sources and to make
  it easy for users to link disparate information
  systems, including computer-based patient
  records,bibliographic databases, factual
  databases, and expert systems.
• There are four UMLS knowledge sources
• UMLS Metathesaurus
• SPECIALIST Lexicon
• UMLS Semantic Network
• UMLS Information Sources Map

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Ontology Conclusions

• Shared ontologies are essential for agent
  communication and knowledge sharing
• Ontology tools and standards are important
• Ontolingua and OKBC are good examples
• XML and RDF may be a next step
• Some large general ontologies are available
• Cyc, WFB, WordNet,
• For more information
• http//www.kr.org/top describes projects
  addressing major ontology construction issues
• Ontology mailing list send mail to
  majordomo_at_cs.umbc.edu with info ontology in
  message body for information.
• ANSI Ad Hoc Group on Ontology Standards
  http//WWW-KSL.Stanford.EDU/onto-std/