ontology components
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Contemporary
ontologies In computer science Computer science deals with the theoretical foundations of information, algorithms and the architectures of its computation as well as practical techniques for their application. Computer science is the study of Algor ...
share many structural similarities, regardless of the
ontology language In computer science and artificial intelligence, ontology languages are formal languages used to construct ontology (information science), ontologies. They allow the encoding of knowledge about specific Field of study, domains and often include rea ...
in which they are expressed. Most ontologies describe individuals (instances), classes (concepts), attributes, and relations.


Overview

Common components of ontologies include: ;Individuals: instances or objects (the basic or "ground level" objects). ;
Class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytical concept used differently f ...
es: sets, collections, concepts, types of objects, or kinds of things. ;
Attributes Attribute may refer to: * Attribute (philosophy), an extrinsic property of an object * Attribute (research), a characteristic of an object * Grammatical modifier, in natural languages * Attribute (computing), a specification that defines a propert ...
: aspects, properties, features, characteristics, or parameters that objects (and classes) can have. ;
Relations Relation or relations may refer to: General uses *International relations, the study of interconnection of politics, economics, and law on a global level *Interpersonal relationship, association or acquaintance between two or more people *Public ...
: ways in which classes and individuals can be related to one another. ;Function terms: complex structures formed from certain relations that can be used in place of an individual term in a statement. ;Restrictions: formally stated descriptions of what must be true in order for some assertion to be accepted as input. ;Rules: statements in the form of an if-then (antecedent-consequent) sentence that describe the logical inferences that can be drawn from an assertion in a particular form. ;Axioms: assertions (including rules) in a
logical form In logic Logic (from Ancient Greek, Greek: grc, wikt:λογική, λογική, label=none, lit=possessed of reason, intellectual, dialectical, argumentative, translit=logikḗ)Also related to (''logos''), "word, thought, idea, argument ...
that together comprise the overall theory that the ontology describes in its domain of application. This definition differs from that of "axioms" in generative grammar and formal logic. In these disciplines, axioms include only statements asserted as ''a priori'' knowledge. As used here, "axioms" also include the theory derived from axiomatic statements. ;
Events Event may refer to: Gatherings of people * Ceremony, an event of ritual significance, performed on a special occasion * Convention (meeting), a gathering of individuals engaged in some common interest * Event management, the organization of event ...
: the changing of attributes or relations. Ontologies are commonly encoded using
ontology language In computer science and artificial intelligence, ontology languages are formal languages used to construct ontology (information science), ontologies. They allow the encoding of knowledge about specific Field of study, domains and often include rea ...
s.


Individuals

Individuals (instances) are the basic, "ground level" 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 (although there are differences of opinion as to whether numbers and words are classes or individuals). Strictly speaking, an ontology need not include any individuals, but one of the general purposes of an ontology is to provide a means of classifying individuals, even if those individuals are not explicitly part of the ontology. In formal extensional ontologies, only the utterances of words and numbers are considered individuals – the numbers and names themselves are classes. In a 4D ontology, an individual is identified by its spatio-temporal extent. Examples of formal extensional ontologies are
BORO BORO (Business Objects Reference Ontology) is an approach to developing Ontologies (computer science), ontological or Semantic data model, semantic models for large complex operational applications that consists of a top ontology as well as a proces ...

BORO
, ISO 15926 and the model in development by the IDEAS Group.


Classes

Classes – concepts that are also called ''type'', ''sort'', ''category'', and ''kind'' – can be defined as an extension or an intension. According to an extensional definition, they are abstract groups, sets, or collections of objects. According to an intensional definition, they are abstract objects that are defined by values of aspects that are constraints for being member of the class. The first definition of class results in ontologies in which a class is a subclass of collection. The second definition of class results in ontologies in which collections and classes are more fundamentally different. Classes may classify individuals, other classes, or a combination of both. Some examples of classes:Note that the names given to the classes mentioned here are entirely a matter of convention. * ''Person'', the class of all people, or the abstract object that can be described by the criteria for being a person. * ''Vehicle'', the class of all vehicles, or the abstract object that can be described by the criteria for being a vehicle. * ''Car'', the class of all cars, or the abstract object that can be described by the criteria for being a car. * ''Class'', representing the class of all classes, or the abstract object that can be described by the criteria for being a class. * ''Thing'', representing the class of all things, or the abstract object that can be described by the criteria for being a thing (and not nothing). Ontologies vary on whether classes can contain other classes, whether a class can belong to itself, whether there is a universal class (that is, a class containing everything), etc. Sometimes restrictions along these lines are made in order to avoid certain well-known
paradox A paradox is a logically self-contradictory statement or a statement that runs contrary to one's expectation. It is a statement that, despite apparently valid reasoning from true premises, leads to a seemingly self-contradictory or a logically un ...

paradox
es. The classes of an ontology may be
extensionalIn philosophy of language In analytic philosophy, philosophy of language investigates the nature of language A language is a structured system of communication used by humans, including speech (spoken language), gestures (Signed language, ...
or
intension In linguistics Linguistics is the science, scientific study of language. It encompasses the analysis of every aspect of language, as well as the methods for studying and modeling them. The traditional areas of linguistic analysis include ...

intension
al in nature. A class is extensional if and only if it is characterized solely by its membership. More precisely, a class C is extensional if and only if for any class C', if C' has exactly the same members as C, then C and C' are identical. If a class does not satisfy this condition, then it is intensional. While extensional classes are more well-behaved and well understood mathematically, as well as less problematic philosophically, they do not permit the fine grained distinctions that ontologies often need to make. For example, an ontology may want to distinguish between the class of all creatures with a kidney and the class of all creatures with a heart, even if these classes happen to have exactly the same members. In most upper ontologies, the classes are defined intensionally. Intensionally defined classes usually have necessary conditions associated with membership in each class. Some classes may also have sufficient conditions, and in those cases the combination of necessary and sufficient conditions make that class a fully ''defined'' class. Importantly, a class can subsume or be subsumed by other classes; a class subsumed by another is called a ''subclass'' (or ''subtype'') of the subsuming class (or ''supertype''). For example, ''Vehicle'' subsumes ''Car'', since (necessarily) anything that is a member of the latter class is a member of the former. The subsumption relation is used to create a hierarchy of classes, typically with a maximally general class like ''Anything'' at the top, and very specific classes like ''2002 Ford Explorer'' at the bottom. The critically important consequence of the subsumption relation is the inheritance of properties from the parent (subsuming) class to the child (subsumed) class. Thus, anything that is necessarily true of a parent class is also necessarily true of all of its subsumed child classes. In some ontologies, a class is only allowed to have one parent (''single inheritance''), but in most ontologies, classes are allowed to have any number of parents (''multiple inheritance''), and in the latter case all necessary properties of each parent are inherited by the subsumed child class. Thus a particular class of animal (''HouseCat'') may be a child of the class ''Cat'' and also a child of the class ''Pet''. A partition is a set of related classes and associated rules that allow objects to be classified by the appropriate subclass. The rules correspond with the aspect values that distinguish the subclasses from the superclasses. For example, to the right is the partial diagram of an ontology that has a partition of the ''Car'' class into the classes ''2-Wheel Drive Car'' and ''4-Wheel Drive Car''. The partition rule (or subsumption rule) determines if a particular car is classified by the ''2-Wheel Drive Car'' or the ''4-Wheel Drive Car'' class. If the partition rule(s) guarantee that a single ''Car'' cannot be in both classes, then the partition is called a disjoint partition. If the partition rules ensure that every concrete object in the super-class is an instance of at least one of the partition classes, then the partition is called an exhaustive partition.


Attributes

Objects in an ontology can be described by relating them to other things, typically ''aspects'' or ''parts''. These related things are often called ''attributes'', although they may be independent things. Each attribute can be a class or an individual. The kind of object and the kind of attribute determine the kind of relation between them. A relation between an object and an attribute express a fact that is specific to the object to which it is related. For example, the
Ford Explorer The Ford Explorer is a range of Sport utility vehicle, SUVs manufactured by Ford Motor Company since the 1991 model year. The first four-door SUV produced by Ford, the Explorer was introduced as a replacement for the two-door Ford Bronco II, Bro ...
object has attributes such as: * Ford Explorer * ''6-speed transmission'' * ''door'' (with as minimum and maximum cardinality: 4) * ' The value of an attribute can be a complex
data type In computer science Computer science deals with the theoretical foundations of information, algorithms and the architectures of its computation as well as practical techniques for their application. Computer science is the study of Alg ...
; in this example, the related engine can only be one of a list of subtypes of engines, not just a single thing. Ontologies are only true ontologies if concepts are related to other concepts (the concepts do have attributes). If that is not the case, then you would have either a
taxonomy Taxonomy (general) is the practice and science of classification of things or concepts, including the principles that underlie such classification. The term may also refer to a specific classification scheme. Originally used only about biological ...
(if
hyponym In linguistics, hyponymy (from Greek language, Greek ὑπό, ''hupó'', "under", and ὄνυμα, ''ónuma'', "name") is a semantics, semantic relation between a hyponym denoting a subtype and a hypernym or hyperonym denoting a supertype. In oth ...
relationships exist between concepts) or a
controlled vocabulary Controlled vocabularies provide a way to organize knowledge for subsequent retrieval. They are used in subject indexing schemes, subject headings, thesaurus (information retrieval), thesauri, Taxonomy (general), taxonomies and other knowledge orga ...
. These are useful, but are not considered true ontologies.


Relationships

Relationships (also known as relations) between objects in an ontology specify how objects are related to other objects. Typically a relation is of a particular type (or class) that specifies in what sense the object is related to the other object in the ontology. For example, in the ontology that contains the concept Ford Explorer and the concept
Ford Bronco The Ford Bronco is a model line of SUVs manufactured and marketed by Ford Motor Company, Ford. The first sport-utility vehicle developed by the company, five generations of the Bronco were sold from the 1966 to 1996 model years; a sixth generati ...

Ford Bronco
might be related by a relation of type . The full expression of that fact then becomes: * Ford Explorer ''is defined as a successor of '': Ford Bronco This tells us that the Explorer is the model that replaced the Bronco. This example also illustrates that the relation has a direction of expression. The inverse expression expresses the same fact, but with a reverse phrase in natural language. Much of the power of ontologies comes from the ability to describe relations. Together, the set of relations describes the
semantics Semantics (from grc, σημαντικός ''sēmantikós'', "significant") is the study of reference Reference is a relationship between objects in which one object designates, or acts as a means by which to connect to or link to, another o ...
of the domain. The set of used relation types (classes of relations) and their subsumption hierarchy describe the expression power of the language in which the ontology is expressed. An important type of relation is the
subsumption Subsumption may refer to: * A minor premise in symbolic logic (see syllogism) * The Liskov substitution principle in object-oriented programming * Subtyping in programming language theory * Subsumption architecture in robotics * A Hierarchy#Subsum ...
relation (''is-a- superclass-of'', the converse of ''
is-a In knowledge representation Knowledge representation and reasoning (KR², KR&R) is the field of artificial intelligence Artificial intelligence (AI) is intelligence demonstrated by machines, unlike the natural intelligence human intelligence, ...
'', ''is-a-subtype-of'' or ''is-a- subclass-of''). This defines which objects are classified by which class. For example, we have already seen that the class Ford Explorer ''is-a-subclass-of'' 4-Wheel Drive Car, which in turn ''is-a-subclass-of'' Car. The addition of the is-a-subclass-of relationships creates a
taxonomy Taxonomy (general) is the practice and science of classification of things or concepts, including the principles that underlie such classification. The term may also refer to a specific classification scheme. Originally used only about biological ...
; a tree-like structure (or, more generally, a
partially ordered set upright=1.15, Fig.1 The Hasse diagram of the Power set, set of all subsets of a three-element set \, ordered by set inclusion, inclusion. Sets connected by an upward path, like \emptyset and \, are comparable, while e.g. \ and \ are not. In mathem ...
) that clearly depicts how objects relate to one another. In such a structure, each object is the 'child' of a 'parent class' (Some languages restrict the is-a-subclass-of relationship to one parent for all nodes, but many do not). Another common type of relations is the
mereology In logic Logic (from Ancient Greek, Greek: grc, wikt:λογική, λογική, label=none, lit=possessed of reason, intellectual, dialectical, argumentative, translit=logikḗ)Also related to (''logos''), "word, thought, idea, argument, a ...
relation, written as ''part-of'', that represents how objects combine to form composite objects. For example, if we extended our example ontology to include concepts like Steering Wheel, we would say that a "Steering Wheel is-by-definition-a-part-of-a Ford Explorer" since a steering wheel is always one of the components of a Ford Explorer. If we introduce meronymy relationships to our ontology, the hierarchy that emerges is no longer able to be held in a simple tree-like structure since now members can appear under more than one parent or branch. Instead this new structure that emerges is known as a
directed acyclic graph In mathematics, particularly graph theory, and computer science, a directed acyclic graph (DAG or dag ) is a directed graph with no Cycle graph#Directed cycle graph, directed cycles. That is, it consists of Vertex (graph theory), vertices and edg ...

directed acyclic graph
. As well as the standard is-a-subclass-of and is-by-definition-a-part-of-a relations, ontologies often include additional types of relations that further refine the semantics they model. Ontologies might distinguish between different categories of relation types. For example: * relation types for relations between classes * relation types for relations between individuals * relation types for relations between an individual and a class * relation types for relations between a single object and a collection * relation types for relations between collections Relation types are sometimes domain-specific and are then used to store specific kinds of facts or to answer particular types of questions. If the definitions of the relation types are included in an ontology, then the ontology defines its own ontology definition language. An example of an ontology that defines its own relation types and distinguishes between various categories of relation types is the
Gellish Gellish is an ontology language for data storage and communication, designed and developed by Andries van Renssen since mid-1990s. It started out as an engineering modeling language ("Generic Engineering Language", giving it the name, "Gellish") but ...
ontology. For example, in the domain of automobiles, we might need a ''made-in'' type relationship which tells us where each car is built. So the Ford Explorer is ''made-in''
Louisville Louisville (, , ) is the largest city A city is a large human settlement In geography, statistics and archaeology, a settlement, locality or populated place is a community in which people live. The complexity of a settlement can ra ...
. The ontology may also know that Louisville is-located-in
Kentucky Kentucky ( , ), officially the Commonwealth of Kentucky, is a U.S. state, state in the Southeastern United States, Southeastern region of the United States, bordered by Illinois, Indiana, and Ohio to the north; West Virginia and Virginia to ...
and Kentucky is-classified-as-a state and is-a-part-of the
U.S. The United States of America (USA), commonly known as the United States (U.S. or US), or America, is a country primarily located in North America North America is a continent entirely within the Northern Hemisphere and almost all ...

U.S.
Software using this ontology could now answer a question like "which cars are made in the U.S.?"


Notes

{{reflist Ontology (information science)