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Semantic Web Technologies
LectureDr. Harald Sack
Hasso-Plattner-Institut für IT Systems EngineeringUniversity of Potsdam
Winter Semester 2012/13
Lecture Blog: http://semweb2013.blogspot.com/This file is licensed under the Creative Commons Attribution-NonCommercial 3.0 (CC BY-NC 3.0)
Semantic Web Technologies , Dr. Harald Sack, Hasso-Plattner-Institut, Universität Potsdam
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Fundamental Questions of Ontology
1.What does it mean for a being to be?• When are two things identical?• Is everything that exists also real?• Does something exist, if it is only possible?• Are there non-existing things?
2.What categories of objects do exist?• Do things exist that are only unique or
only multiple (Universalia)?• Do things exist that are unilaterally
dependent of others (Substances)?• Of which sort is this dependency
(Causality)?• Do necessary properties exist
(Essences)?• How do composed things relate to their
Semantic Web Technologies , Dr. Harald Sack, Hasso-Plattner-Institut, Universität Potsdam
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Rudolf Goclenius the Elder(1547-1628)
•„Ontology“ first turned up in 1606 („ontologia“ in Latin) by Jacob Lorhard in his book „Ogdoas Scholastica“
•In German language the term „Ontologie“ first turns up in 1613 in Rudolf Göckel‘s „Lexicon philosophicum“
Ontology: where does the term come from?
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Christian Wolff(1679-1754)
•Christian Wolff named Ontology a philosophical discipline as Part of Metaphysics, more specific of general metaphysics (metaphysica generalis)
•Traditional Ontology deals with the relation of the ,being‘ to „existence“•ontological difference
(Heidegger)•Division of „Existenciality“ and
„Categoriality“
Ontology: where does the term come from?
Ontology in Classic Greek Philosophy
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Raffael: Die Schule von Athen, 1510-11
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Ontology in Classical Greek Philosophy
• Parmenides (6th-5th Century BC)
• Fundamental Question:„What are the fundamental categories of existence?“
Parmenides (Peri Physeios, 7.1-7.4)
Parmenides(ca. 535-470 BC)
„...Denn es ist unmöglich, dass dies zwingend erwiesen wird: es sei Nichtseiendes; Vielmehr halte du von diesem Wege der Forschung den Gedanken fern, Und es soll dich nicht vielerfahrene Gewohnheit auf diesen Weg zwingen, Walten zu lassen das blicklose Auge und das dröhnende Gehör Und die Zunge, nein mit dem Denken bring zur Entscheidung die streitreiche Prüfung, Die von mir genannt wurde.“
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Raffael: Die Schule von Athen, 1510-11
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• Socrates / Platon (5th-4th Century BC)
Dirk Cürsgen, in: Schäfer (Hrsg.), Platon-Lexikon, 2007, S. 102
Ontology in Classical Greek Philosophy
„Die Idee/Form (gr. idea, eidos) ist ein wesenhaft eingestaltiges immer Seiendes, das in den vielen Einzelnen zur Darstellung kommt. Demnach sind bestimmte nur durch die Vernunft zugängliche Entitäten dem Sein und der Erkenntnis nach gegenüber konkreten, sinnlich wahrnehmbaren Einzelgegenständen vorrangig und stehen als seinsbegründende Urbilder in einer bestimmten Beziehung zu diesen. Die Ideen sind wahrhaft seiende, undingliche bloß denkbare reine Einheiten von Bestimmungen, Prinzipien oder Gegenstandsklassen, die allem Einzelnen, das unter sie fällt, das vermitteln, was es als es selbst sein lässt und wodurch es als bestimmt erkennbar ist.
Semantic Web Technologies , Dr. Harald Sack, Hasso-Plattner-Institut, Universität Potsdam
2016 • Platon (5th-4th Century BC)
•Division between
• concept / class(idea, ,Noosphere‘, word of idea)
and
• entity (instance)(real world objects, merely „shadows“ of ideas)
•Hierarchy of ideas
Ontology in Classical Greek Philosophy
Platon(427-347 BC)
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Platon(427-347 BC)
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(Abbildung aus: Osborne, Philosophie - Eine Bildergeschichte für Einsteiger)
Platons Cave Parabel• from Platon‘s ,Politeia‘, 7. Book, ca. 370 BC
Ontology in Classical Greek Philosophy
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Aristoteles (De Interpretatione I, 16)
• Aristotle (Metaphysics IV, 1)defines a system of general categories for classification of all things about which statements can be made
substance or quantity or qualification or a relative or where or when or being-in-a-position or having or doing or being-affected.
Ontology in Classical Greek Philosophy
Aristotle(384-322 BC)
„Nun sind die (sprachlichen) Äußerungen unserer Stimme Symbole für das, was (beim Sprechen) unserer Seele widerfährt, und unsere schriftlichen Äußerungen sind wiederum Symbole für die (sprachlichen) Äußerungen unserer Stimme. Und wie nicht alle Menschen mit denselben Buchstaben schreiben, so sprechen sie auch nicht dieselbe Sprache. Die seelischen Widerfahrnisse aber, für welche dieses (Gesprochene und Geschriebene) an erster Stelle ein Zeichen ist, sind bei allen Menschen dieselben; und überdies sind auch schon die Dinge, von denen diese (seelischen Widerfahrnisse) Abbildungen sind, für alle dieselben.“
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Aristotle(384-322 BC)
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2316Aristotle defines a system of general categories for classification of all things about which statements can be made
Aristotle‘s Categories
Ontology in Classical Greek Philosophy
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Aristotle(384-322 BC)
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wikipedia.org
Ontology in Classical Greek Philosophy
Aristotle defines a system of general categories for classification of all things about which statements can be made
Aristotle‘s Categories
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Aristotle(384-322 BC)
Syllogisms
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• συλ-λογισμός [griech.] add up, logical conclusion•Core of Aristotelian logic• Inference rules, all built according to the same
pattern
Ontology in Classical Greek Philosophy
major premise All humans are mortal
minor premise All Greeks are humans
conclusion All Greeks are mortal
subject predicate
major termminor term middle term
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Ontology in Late Antiquity and Early Middle Ages
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Aristotle‘s Categories Revisited
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•Porphy of Tyros explains in his textbook „Isagoge“ the Aristotelian categories for beginners
• In the Late Middle Ages the so-called„arbor porphyriana“ (Tree of Porphyry or „Tree of Knowledge“) is created as visualization of the Aristotelian categories
• classic epistemologic ordering system, according to the semantics of botany
Ontology in Late Antiquity andearly Middle Ages
Porphyry of Tyros(234-?? AD)
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Ontologie in der klassischen griechischen Philosophie
wikipedia.org
Porphyry of Tyros(234-?? AD)
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Porphyry of Tyros(234-?? AD)
Ontology in Late Antiquity andearly Middle Ages
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Ontology in the Middle Ages
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3116 Medieval Scholasticism (12th-14th century)
• Thomas Aquinas• Anselm of Canterbury• William of Ockham
Thomas Aquinas(1225-1274)
Anselm of Canterbury(1033-1109)
William of Ockham(1288-1349)
• Problem of Universals: „Do Universals (Ideas) possess an ontological (real) existency?“
• Realism „Universals are real!“ vs.• Nominalism „Universals are nothing but words and symbols!“
Ontology in the Middle Ages
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3216 Occam‘s Razor
• „Principle of Simplicity“ from Scholasticism• Among competing hypotheses, the one that makes
the fewest assumptions should be selected• A theory is simple, if
• it contains as few variables and hypotheses as possible
• that relate clearly logical with each other and • the subject to be explained can be logically
deduced.
William of Ockham(1288-1349)
„Entia non sunt multiplicanda sine necessitate.“
Ontology in the Middle Ages
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• Christian doctrine of trinity as example for a medieval ontology
Ontology in the Middle Ages
Tritheism of Roscellin of Compiégne (ca. 1050-1125)
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Ramon Lull(Raimundus Lullus)
(1232-1316
• Ramon Lull applies ontology in the sense of the Aristotelian system of categories within his „Tree of Nature and Logic“ as first predecessor of a logical machine
• Logic according to Ramon Lull is „the art and the science to distinguish between truth or lie with the help of reason, to accept truth and to reject lie.“
Ontology in the Middle Ages
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Arbor naturalis et logicalis, aus„Ars Magna“, um 1275
Ramon Lull(Raimundus Lullus)
(1232-1316
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3616 Ontology in the Age of Enlightenment
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John Wilkins(1614 – 1672)
• John Wilkins finds out, that the inaccuracy of natural language impedes scientific progress
• He developed the idea of a universal philosophical Language with the goal to represent the entire knowledge of the universe.
Ontology in the Age of Enlightenment
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John Wilkins(1614 – 1672)
,A Doctor counted very ableDesignes that all Mankynd converse shall,
Spite o' th' confusion made att Babell,By Character call'd Universall.
How long this character will be learning,That truly passeth my discerning.‘
(Ballad of Gresham College, 1663)
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Gottfried Wilhelm Leibniz(1646 – 1716)
• Gottfried Wilhelm Leibnitz develops the idea of „characteristic numbers“ as a model for Aristotelian Logic, in the hope to solve logical problems with the help of a calculus.
„Wenn man Charaktere oder Zeichen finden könnte, die alle unsere Gedanken genauso rein und klar ausdrücken könnten wie die Arithmetik Zahlen oder die Analytische Geometrie Linien ausdrückt, dann könnte man in allen Angelegenheiten, soweit sie dem rationalen Denken zugänglich sind, das tun, was man in der Arithmetik und Geometrie tut. “
Ontology in the Age of Enlightenment
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Ontologie in der Philosophie der Aufklärung
• Immanuel Kant turns himself towards Epistemology (Critique of „Pure Reason“)
• Fundamental Question: „How can our mind perceive the world?“
Immanuel Kant(1724-1804)
„Kategorien sind nach Kant apriorisch und unmittelbar gegeben. Sie sind Werkzeuge des Urteilens und Werkzeuge des Wahrnehmens. Als solche dienen sie nur der Anwendung und haben keine Existenz. Sie bestehen somit nur im menschlichen Verstand. Sie sind nicht an Erfahrung gebunden. Durch ihre Unmittelbarkeit sind sie auch nicht an Zeichen gebunden. Kants erkenntnistheoretisches Ziel ist es, über die Bedingungen der Geltungskraft von Urteilen Auskunft zu geben.“
R. Eisler. Kant-Lexikon : Nachschlagewerk zu Kants samtlichen Schriften / Briefen und handschriftlichem Nachlass.. Olms, Hildesheim, 1961
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• Immanuel Kant turns himself towards Epistemology (Kritik an der „reinen Vernunft“)
• Epistemology: Categories are pure concepts of understanding
Quantity Quality Relation Modality
Unity RealitySubstance &
AccidentPossibility
Plurality Negation Cause & Effect Existence
Totality Limitation Reciprocity Necessity
Immanuel Kant(1724-1804)
Ontology in the Age of Enlightenment
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The Concept of Ontologyin Computer Science
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4416 "An ontology is an explicit, formal specification of a shared conceptualization. The term is borrowed from philosophy, where an Ontology is a systematic account of Existence. For AI systems, what ‘exists’ is that which can be represented.“
Thomas R. Gruber: A Translation Approach to Portable Ontology Specifications.Knowledge Acquisition, 5(2):199-220, 1993.
conceptualization: abstract Model (Domain, identified relevant concepts, relations)explicit: Meaning of all concepts must be definedformal: machine understandableshared: consensus about ontology
tomgruber.org
The Concept of Ontology in Computer Science
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Conceptualization
Symbol Object
Ogden, Richards: The Meaning of Meaning: A Study of the Influence of Language upon Thought and of the Science of Symbolism (1923)
„Jaguar“
Concept
apply sharedconcept
stands for
refers tosymbolizes
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How should we represent
Ontologies?
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4716 Ontologies - Components and Models
• Classes, Relations and Instances
• Classes represent concepts• Classs are described via attributs• Attributs are Name Value pairs
Address• given name <string>• family name <string>• street <string>• ZIP code <int>• city <string>• …
„The address contains the name, title, and place of residence of the person addressed“
Informal Description
Semi informal Description
How should we represent Ontologies?
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Forms of Modelling
• informal Description in natural language
• semi informal structured description in controlled batural language
• semi formal description in artificial formal defined language
• formal description in well-defined terms with given formal semantics, incl. proof of correctness and completeness.
How should we represent Ontologies?
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• Classes are related to other classes
• Relations are special attributes, whose values are objects of (other) classes
AddressPerson
Professor Student
is subclass of
Lecture
gives visits
Course
Formal Definition: Sets m1,…mn Relation R ⊆ m1 x … x mn
has an
How should we represent Ontologies?
is subclass of
is subclass of
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• For Relations and Attributs Rules (Constraints) can be defined that determine allowed values
AddressPersonhas
Professor Student
1:n
1:1
Woman
Man
≠
Frau ⋂ Mann = ∅
How should we represent Ontologies?
Constraint
is subclass of
is subclass of
is subclass of is subclass of
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• Classes, relations, and constraints can be put together to form statements / assertions
• Special Case: formal Axioms
• Axioms describe knowledge that can‘t be expressed simply with the help of other existing components.
Example: • „it is not possible to give two courses at the
same time“
How should we represent Ontologies?
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• Instances describe individuals of an ontology
Course
Seminar
is subclass of
Bachelor SeminarSW enabled Applications
Thursday11.00 pm A-2.1
Individuals (instances) are the basic components of an ontology. The individuals in an ontology may include concrete objects such as people, animals, tables, automobiles, molecules, and planets, as well as abstract individuals such as numbers and words.
is a
How should we represent Ontologies?
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How do I define a
formal model o
f an ontology?
-
Knowledge Represen
tation and
Ontology Descript
ion Languages
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5416 • in propositional logic the world consists simply of facts and nothing else (statements of assertions)
• Example for propositional logic assertions and deductions:
• If it rains, the road will get wet.• If the moon is made out of green cheese, then cows can fly.• If Oliver is in love, then he will be happy.
• The world consists out of objects and properties that distinguish one objectr from another.
• Between objects are relations. Some relations are unique, i.e. functions.
Propositional Logic
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5516• In First Order Logic (FOL) quantors allow assertions about sets
of objects, without naming the objects explicitely.
• All humans are mortal.• Socrates is a human.• Socrates is mortal.
• FOL is perfectly suited for the description of ontologies, but...• FOL is rather expressive,• therefore also rather bulky for modelling,• difficult to achieve consense in modelling and• rather complex to proof (correctness and completeness of
assertions)
First Order Logic
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Description Logics
Description Logics are a family of languages for knowledge representation. Most description logics are a subset of First Order Logic, but in difference to FOL most description logics decidable. Therefore, it is possible to make logical deductions based on description logics, i.e. to create new knowledge from existing knowledge.
Lecture
Lecture„Semantic Web Technologies“
TBox terminological knowledge Knowledge about concepts of a domain (classes, attributes, relations…)
ABox assertional knowlegde knowledge about instances / entities
Knowledge Base
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• Expressivity is limited:• Satisfiability and Subsumption is decidable and • (preferably) of low complexity• Syntax: participatesAt(Alice, SemanticWeb)
Description Logics
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5916 • Fundamental operators:
• Conjunction (⊓),
• Disjunction (⊔),
• Negation (⌐)
• restricted form of Quantification (∀,∃)
• represents Basic Description Logic ALC• Attributive Language with Complement
Description Logics
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Attributive Language with Complement - ALC• Atomic Types• concept names A, B, ... • special concepts
• ⊤ - Top (universal concept)• ⊥ - Bottom concept
• role names R,S, ...
• Constructors • Negation: ¬C • Conjunction: C ⊓ D• Disjunction: C ⊔ D• Existential Quantor: ∃R.C• Universal Quantor: ∀R.C
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6116 • Class Relations• Inclusion C ⊑ D
• E.g., Man ⊑ Human• Equality C ≣ D
• E.g., Frau ≣ Woman
• Class Constructors
• E.g., Seminarist ≡ Person ⊓ (∃participatesAt.Seminar ⊔ ∃givesLecture.Seminar)
Attributive Language with Complement - ALC
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6216 • Terminological Knowledge (TBox)• Axioms describing the structure of the represented domain
(conceptional schema)• Human ⊑ ∃hasParents.Human
Orphan ≣ Human ⊓ ¬∃hasParents.Alive
• Assertional Knowledge (ABox)• Axioms describing specific situations (data)
Qualified Cardinality Restriction (≤ n R.C) (≥ n R.C) QQ
Description Logics
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6416 • Semantics is determined via Interpretation (ΔI, I)
• ΔI … Domain of Discourse, ΔI ≠ ∅
• Interpretation Function:
• I :A → AI ⊆ ΔI , A ... atomic concept
• I :R → RI ⊆ ΔI x ΔI , R … atomic Relation
┬I = ΔI
⊥I = ∅(¬A)I = ΔI \ AI (C Π D)I = CI ∩ DI (∀ R.C)I = {a ∈ ΔI | ∀ b.<a,b> ∈ RI ⇒ b ∈ CI}(∃ R.┬)I = {a ∈ ΔI | ∃ b.<a,b>∈ RI}
Description Logics
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• Ontologies can also be modelled via database or softwaremodelling technologies, as e.g.
• UML, ER-Model, …
Seminar
- Titel: String - Semester: String - Begin: Date - End: Date - …
Person
- GivenName: String - FamilyName: String - …
participatesAtnn
givesLecture1n
How should we represent Ontologies?
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• Formalisms and models limit the knowledge that can be represented by them
• e.g. UML, ER, SQL are not able to represent complex logical constraints and logical inference
• AI-based languages (e.g. Ontolingua, LOOM, OCML, FLogic,…) and Ontology Markup Languages (z.B. RDFS, DAML+OIL, OWL, …) are better suited for the represenation of ontologies.
How should we represent Ontologies?
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Ontology Types
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Ontology Types and Categories
general, cross domain ontologies (represent very general concepts as e.g., Time, Space, Event independent of a specific domain or problem.)
Domain Ontology Task Ontology
specialized ontology focussed on a specific task and domain
Application Ontology
(according to Guarino,1998)
fundamental concepts according to a general activity or task.
fundamental concepts according to ageneric domain.
Top-Level Ontology(Upper Ontology,
Foundation Ontology)
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Ontology Types and Categories
general, cross domain ontologies (represent very general concepts as e.g., Time, Space, Event independent of a specific domain or problem.)
Domain Ontology Task Ontology
specialized ontology focussed on a specific task and domain
Application Ontology
(according to Guarino,1998)
fundamental concepts according to a general activity or task.fundamental concepts according to a
generic domain.
Top-Level Ontology(Upper Ontology,
Foundation Ontology)
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Ontology Types and Categories
general, cross domain ontologies (represent very general concepts as e.g., Time, Space, Event independent of a specific domain or problem.)
Domain Ontology Task Ontology
specialized ontology focussed on a specific task and domain
Application Ontology
(according to Guarino,1998)
fundamental concepts according to a general activity or task.
Top-Level Ontology(Upper Ontology,
Foundation Ontology)
fundamental concepts according to ageneric domain.
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Ontology Types and Categories
general, cross domain ontologies (represent very general concepts as e.g., Time, Space, Event independent of a specific domain or problem.)
Domain Ontology Task Ontology
specialized ontology focussed on a specific task and domain
Application Ontology
(according to Guarino,1998)
fundamental concepts according to a general activity or task.
fundamental concepts according to ageneric domain.
Top-Level Ontology(Upper Ontology,
Foundation Ontology)
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Ontology Types and Categories
general, cross domain ontologies (represent very general concepts as e.g., Time, Space, Event independent of a specific domain or problem.)
Domain Ontology Task Ontology
specialized ontology focussed on a specific task and domain
Application Ontology
(according to Guarino,1998)
fundamental concepts according to a general activity or task.
fundamental concepts according to ageneric domain.
Top-Level Ontology(Upper Ontology,
Foundation Ontology)
Dienstag, 13. November 12
Semantic Web Technologies , Dr. Harald Sack, Hasso-Plattner-Institut, Universität Potsdam
7316
lightweight ontologies heavyweight ontologies
controlledVocabulary
(according to Lassila/McGuinnes, 2001)
Thesauri
Glossaries informalIS-A
formalIS-A
formalInstance
Frames
ValueRestrictions
generallogical
Constraints
Disjunctness,Inversiveness,Part-of…
Expressivity +-
Ontology Types and Categories
informal formal
Folksonomiesformal
Taxonomies
LogicProgramming Description
Logics
First OrderLogicsData
Dictionaries
Terms
Dienstag, 13. November 12
Semantic Web Technologies , Dr. Harald Sack, Hasso-Plattner-Institut, Universität Potsdam
7416 • Controlled Vocabulary:finite list of terms (e.g. catalogue)
• Glossary: finite list of terms including an informal definition of their semantics in natural language
Semantic Web Technologies , Dr. Harald Sack, Hasso-Plattner-Institut, Universität Potsdam
7516Thesaurus
Underpants
Knickers
Synonyms
Panties
Bloomers
Briefs
Boxershorts
narrower concept
Underwearbroader concept
Undershirt
Association
Ontology Types and Categories
Dienstag, 13. November 12
Semantic Web Technologies , Dr. Harald Sack, Hasso-Plattner-Institut, Universität Potsdam
7616 Taxonomies
• also classification schema, Nomenclature, …• in science most times classification into (mono-)hierarchical sets
(classes, subclasses, ...)• (also) subject of biology:
• the arrangement of organisms into a classification according to similarities
Taxonomy: Definition of a hierarchical system of groups (from [greek] τασσεῖν (tassein) = to arrange and νόµος (nomos) = method) ...
Ontology Types and Categories
Dienstag, 13. November 12
Semantic Web Technologies , Dr. Harald Sack, Hasso-Plattner-Institut, Universität Potsdam
7716
• Carl v. Linné (ca. 1740) created a simple classification schema for organisms that is still used today
Carl v. Linné(1707-1778)
Des Ritters Carl von Linné vollständiges Natursystem, 1778Dienstag, 13. November 12
Semantic Web Technologies , Dr. Harald Sack, Hasso-Plattner-Institut, Universität Potsdam
7816
lightweight ontologies heavyweight ontologies
controlledVocabulary
(according to Lassila/McGuinnes, 2001)
Thesauri
Terms/Glossary
informalis-a
formalis-a
formalInstance
Frames
ValueRestrictions
generallogical
Constraints
Disjunctness,Inversiveness,Part-of…
Expressivity +-
Ontology Types and Categories
Dienstag, 13. November 12
Semantic Web Technologies , Dr. Harald Sack, Hasso-Plattner-Institut, Universität Potsdam
7916 • informal IS-A-Hierarchy: explicite hierarchy of classes, subclass relations are not strict(e.g. index of a library)
• formal IS-A-Hierarchy: explicite hierarchy of classes, suclass relations are strict
• formale instance: explicite class hierarchy, besides subclass relations also instance-of relations are allowed
Ontology Types and Categories
Dienstag, 13. November 12
Semantic Web Technologies , Dr. Harald Sack, Hasso-Plattner-Institut, Universität Potsdam
8016
Ontologies as Interpretation of Reality
various categories of animals from "a certain chinese encyclopedia" according to Jorge Luis Borges:
• Those that belong to the emperor• Embalmed ones• Those that are trained• Suckling pigs• Mermaids (or Sirens)• Fabulous ones• Stray dogs• Those that are included in this classification• Those that tremble as if they were mad• Innumerable ones• Those drawn with a very fine camel hair brush• Et cetera• Those that have just broken the flower vase• Those that, at a distance, resemble flies Jorge Luis Borges
(1899-1986)
Jorge Luis Borges: The Analytical Language of John Wilkins (1942)