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 Ontologies are used to capture knowledge about some domain of interest. ◦ An ontology describes the concepts in the domain and also the relationships that hold between those concepts.  Standard ontology languages is OWL from the World Wide Web Consortium (W3C)

 Complex concepts be built up in definitions out of simpler concepts.  The logical model allows the use of a reasoner which can check whether or not all of the statements and definitions in the ontology are mutually consistent and can also recognize which concept fit under which definitions.

 Individuals (instances)  Properties(slots in Protégé, relations in UML, attributes)  Classes(concepts)

 Individuals, represent objects in the domain in which we are interested.

 Properties are binary relations on individuals

 OWL classes are interpreted as sets that contain individuals. Classes may be organized into a superclass-subclass hierarchy (taxonomy).

 classes are disjoint, so that an individual (or object) cannot be an instance of more than one of these classes.

Figure 4.10: Class Hierarchy

 Object properties  Datatype properties  Annotation properties

 Inverse Property: ◦ Each object property may have a corresponding inverse property. If some property links individual a to individual b then its inverse property will link individual b to individual a.

 Functional Properties(single valued properties, features) ◦ If a property is functional, for a given individual, there can be at most one individual that is related to the individual via the property.

 Inverse Functional Properties ◦ If a property is inverse functional then it means that the inverse property is functional.

 Transitive Properties ◦ If a property is transitive, and the property relates individual a to individual b, and also individual b to individual c, then we can infer that individual a is related to individual c via property P.

 Symmetric Properties ◦ If a property P is symmetric, and the property relates individual a to individual b then individual b is also related to individual a via property P.

 Asymmetric properties ◦ If a property P is asymmetric, and the property relates individual a to individual b then individual b cannot be related to individual a via property P.

 Reflexive properties ◦ A property P is said to be reflexive when the property must relate individual l to itself.

 Irreflexive properties  If a property P is irreflexive, it can be described as a property that relates an individual a to individual b,where individual a and individual b are not the same.

 Properties may have a domain and a range specified.  Properties link individuals from the domain to individuals from the range.  It is important to realize that in OWL domains and ranges should not be viewed as constraints to be checked. They are used as `axioms' in reasoning.

 Property Restrictions  Existential (∃) and Universal Restrictions (∀)

 Reasoners are sometimes called classifiers. Classification, however, is not the only inference service provided by reasoners. For instance, a reasoner also performs consistency checking which is different from classification. Hence, the preferred term is reasoner.  Protégé 4 allows different OWL reasoners to be plugged in, the reasoner shipped with Protégé is called Fact++. The ontology can be `sent to the reasoner' to automatically compute the classification hierarchy, and also to check the logical consistency of the ontology.

Figure 4.42: The Class ProbeInconsistentTopping found to be inconsistent by the reasoner

 Necessary conditions can be read as, If something is a member of this class then it is necessary to fulfill these conditions".  If class A is described using necessary conditions, then we can say that if an individual is a member of class A it must satisfy the conditions. We cannot say that any (random) individual that satisfies these conditions must be a member of class A. However, if class A is now defined using necessary and sufficient conditions, we can say that if an individual is a member of the class A it must satisfy the conditions and we can now say that if any (random) individual satisfies these conditions then it must be a member of class A.  Necessary conditions are simply called Superclasses in Protégé 4. Necessary and sufficient condition are called Equivalent classes.

For example, we might dene a class DaysOfTheWeek to contain the individuals (and only the individuals) Sunday, Monday, Tuesday, Wednesday, Thursday, Friday and Saturday.

 To restrict the relationships for a given property to individuals that are members of a specific class we must use a universal restriction.  Universal restrictions are given the ∀. They constrain the relationships along a given property to individuals that are members of a specific class.

 A closure axiom on a property consists of a universal restriction that acts along the property to say that it can only be filled by the specified fillers.  For example, the closure axiom on the hasTopping property for MargheritaPizza is a universal restriction that acts along the hasTopping property, with a filler that is the union of MozzarellaTopping and also TomatoTopping.

Figure 4.51: Class Description View: Margherita Pizza With a Closure Axiom for the hasTopping property

 A covering axiom consists of two parts:The class that is being `covered', and the classes that form the covering.

 For a given property P, a Minimum Cardinality Restriction species the minimum number of P relationships that an individual must participate in. A Maximum Cardinality Restriction specifies the maximum number of P relationships that an individual can participate in.

 Datatype properties link an individual to an XML Schema Datatype value or an rdf literal.  In other words, they describe relationships between an individual and data values.

 OWL allows classes, properties, individuals and the ontology itself (technically speaking the ontology header) to be annotated with various pieces of information/meta-data. These pieces of information may take the form of auditing or editorial information.

 Create Pizza Ontology by Protégé.