31 
 
 
The restrictions for a class are displayed and edited using the Class Description View shown in Figure 
4.17. The Class Description View holds most of the information used to describe a class. The Class 
Description View is a powerful way of describing and defining classes. It is one of the most important 
differences between describing classes in OWL and in other models such as most object-oriented 
programming languages. In other models there is no formal definition that describes why one class is a 
subclass of another, in OWL there is. Indeed, the OWL classifier can actually redefine the class hierarchy 
based on the logical restrictions defined by the user. We will see an example of this later in the tutorial.  
 
 
 
 
 
 
4.10.2 Existential Restrictions 
An existential restriction describes a class of individuals that have at least one (some) relationship along a 
specified property to an individual that is a member of a specified class or datatype. For example, 
hasBase some PizzaBase describes all of the individuals that have at least one relationship along the 
hasBase property to an individual that is a member of the class PizzaBase — in more natural English, 
all of the individuals that have at least one pizza base. 
 
Exercise 13: Add a restriction to Pizza that specifies a Pizza must have a PizzaBase 
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1. Select Pizza from the class hierarchy on the Classes tab.    
2. Click on the Add icon (+) next to the SubClass Of field in the Description view for Pizza.  
3. This will bring up a new window with several tab options to define a new restriction. Select the Object 
restriction creator. This tab has the Restricted property on the left and the Restriction filler on the right. 
4. Expand the property hierarchy on the left and select hasBase as the property to restrict.  Then in the 
Restriction filler on the right select the class PizzaBase. Finally, the Restriction type at the bottom should 
be set to Some (existential). This should be the default so you shouldn’t have to change anything but 
double check that this is the case. Your window should look like figure 4.16 now.  
5. When your UI looks like figure 4.16 click on the OK button. That should close the window. Run the 
reasoner to make sure things are consistent.  Your main window should now look like figure 4.17. 
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Restrictions are also called axioms in OWL. This has the same meaning as in logic. An axiom is a 
logical formula defined by the user rather than deduced by the reasoner.  As described above, 
in Protégé all axioms are shown in normal font whereas all inferences inferred by the reasoner 
are highlighted in yellow.