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What are OWL Ontologies?

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. Different ontology languages provide different facilities. The most recent development in standard ontology languages is OWL from the World Wide Web Consortium (W3C)1 . Like Prot´eg´e, OWL makes it possible to describe concepts but it also provides new facilities. It has a richer set of operators - e.g. intersection, union and negation. It is based on a different logical model which makes it possible for concepts to be defined as well as described. Complex concepts can therefore be built up in definitions out of simpler concepts. Furthermore, 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 recognise which concepts fit under which definitions. The reasoner can therefore help to maintain the hierarchy correctly. This is particularly useful when dealing with cases where classes can have more than one parent.

3.1 Components of OWL Ontologies

OWL ontologies have similar components toProt´eg´e frame based ontologies. However, the terminology used to describe these components is slightly different from that used in Prot´eg´e. An OWL ontology consists of Individuals, Properties, and Classes, which roughly correspond to Prot´eg´e frames Instances, Slots and Classes.

3.1.1 Individuals

Individuals, represent objects in the domain in which we are interested 2 . An important difference between Prot´eg´e and OWL is that OWL does not use the Unique Name Assumption (UNA). This means that two different names could actually refer to the same individual. For example, “Queen Elizabeth”, “The Queen” and “Elizabeth Windsor” might all refer to the same individual. In OWL, it must be explicitly stated that individuals are the same as each other, or different to each other — otherwise they might be the same as each other, or they might be different to each other. Figure 3.1 shows a representation of some individuals in some domain—in this tutorial we represent individuals as diamonds in diagrams. 1http://www.w3.org/TR/owl-guide/ 2Also known as the domain of discourse.

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Matthew FluffyGemma Fido Figure 3.1: Representation Of Individuals

Individuals are also known as instances. Individuals can be referred to as being ‘instances of classes’. Figure 3.2: Representation Of Properties

3.1.2 Properties

Properties are binary relations3 on individuals - i.e. properties link two individuals together4 . For example, the property hasSibling might link the individual Matthew to the individual Gemma, or the property hasChild might link the individual Peter to the individual Matthew. Properties can have inverses. For example, the inverse of hasOwner is isOwnedBy. Properties can be limited to having a single value – i.e. to being functional. They can also be either transitive or symmetric. These ‘property characteristics’ are explained in detail in Section 4.8. Figure 3.2 shows a representation of some properties linking some individuals together. Properties are roughly equivalent to slots in Prot´eg´e. They are also known as roles in description logics and relations in UML and other object oriented notions. In GRAIL and some other formalisms they are called attributes. 3A binary relation is a relation between two things. 4Strictly speaking we should speak of ‘instances of properties’ linking individuals, but for the sake of brevity we will keep it simple.

3.1.3 Classes

OWL classes are interpreted as sets that contain individuals. They are described using formal (mathematical) descriptions that state precisely the requirements for membership of the class. For example, the class Cat would contain all the individuals that are cats in our domain of interest.5 Classes may be organised into a superclass-subclass hierarchy, which is also known as a taxonomy. Subclasses specialise (‘are subsumed by’) their superclasses. For example consider the classes Animal and Cat – Cat might be a subclass of Animal (so Animal is the superclass of Cat). This says that, ‘All cats are animals’, ‘All members of the class Cat are members of the class Animal’, ‘Being a Cat implies that you’re an Animal’, and ‘Cat is subsumed by Animal’. One of the key features of OWL-DL is that these superclass-subclass relationships (subsumption relationships) can be computed automatically by a reasoner – more on this later. Figure 3.3 shows a representation of some classes containing individuals – classes are represented as circles or ovals, rather like sets in Venn diagrams. The word concept is sometimes used in place of class. Classes are a concrete representation of concepts. In OWL classes are built up of descriptions that specify the conditions that must be satisfied by an individual for it to be a member of the class. How to formulate these descriptions will be explained as the tutorial progresses.