text: 4.1 Named Classes 
  The main building blocks of an OWL ontology are classes. In Protégé 5, editing of classes can be done in 
  the Entities tab. The Entities tab has a number of sub-tabs. When you select it, the default should be the 
  Class hierarchy view as shown in Figure 4.5.4 All empty ontologies contains one class called owl:Thing. 
  OWL classes are sets of individuals. The class owl:Thing is the class that represents the set containing 
  all individuals. Because of this all classes are subclasses of owl:Thing. 