Why does Pellet produce different results than the Jena OWL reasoner?
There is one fundamental difference between Pellet in Jena and other existing Jena reasoners. Pellet treats the anonymous restrictions defined in an OWL ontology as syntactic expressions and does not return them as answers to any query. Consider the following example:
Class(<a:Person> partial restriction(<a:hasAddress> someValuesFrom(<a:Address>))
Class(<a:Student> partial <a:Person>))
ObjectProperty(a:hasAddress Functional)If this ontology is loaded in an OntModel backed by Pellet, calling Student.listSuperClasses() would not include the restriction in the result. There are several reasons for this behavior: Treating each restriction as a named class would make it harder to reason with the ontology and the results you get in the end are not that useful.
For example, in the above example, the reasoner can also return restriction(<a:hasAddress> minCardinality(1)) or restriction(<a:hasAddress> maxCardinality(1)) or restriction(<a:hasAddress> allValuesFrom(<a:Address>)) as a super class because they are all entailed by the above definition. Once class expressions are considered, there are infinitely many possibilities, e.g. maxCardinality(1) implies maxCardinality(2) and so on.
For this reason, Pellet will not return a restriction in an answer just because it exists in the ontology. However, one needs to get the syntactic definitions in the ontology and all the results Pellet returns are concatenated by the answers from the raw model. This means calling Person.listSuperClasses() for the above example would include the restriction in the results.
Note that all the boolean functions still work as expected. For example, asking the question Student.hasSuperClass( restriction ) where restriction is the anonymous resource corresponding to the someValuesFrom restriction, Pellet will return true.