The Semantic Web - Enabling Semantic Web Services: The Web Service Modeling Ontology - page 31

3.3 The Web Ontology Language OWL

The Web Ontology Language OWL [32] is an expressive ontology language
which extends RDFS. OWL itself consists of three species of increasing ex-
pressiveness:

• OWL Lite. The least expressive of the OWL species. Compared with RDFS

it adds local range restrictions, existential restrictions, simple cardinality
restrictions, equality, and various types of properties (inverse, transitive,
and symmetric).

• OWL DL. Compared with OWL Lite, OWL DL adds full support for (clas-

sical) negation, disjunction, cardinality restrictions, enumerations, and
value restrictions. The element “DL” comes from the resemblance to an
expressive description logic language [5], namely SHOIN ( D ).

• OWL Full. Whereas OWL Lite and OWL DL impose restrictions on the

use of vocabulary and the use of RDF statements, OWL Full does not
have such restrictions. Therefore, OWL Full allows both the specification
of classes-as-instances and the use of language constructs in the language
itself, which thereby modifies the language.

For OWL Lite, it turns out that although there are many syntactic restric-
tions in the language, the actual expressiveness of the language is very close to
the expressiveness of OWL DL [65]. OWL Full is a very expressive language
and, because of the syntactic freedom which is allowed in the language, key
inference problems are undecidable. In the remainder of this section, we shall
focus on OWL DL.
Whereas statements in RDF(S) are triples, statements in OWL DL are ei-
ther axioms or assertions. An axiom is either a class definition, a class axiom,
or a property axiom. Class definitions can be used to define subclass rela-
tionships, as well as certain property restrictions which hold for a particular
class. With class and property axioms, one can express more complex relation-
ships between classes and between properties such as boolean combinations
of class descriptions and transitive, inverse, and symmetric properties. Indi-
vidual assertions can be used to express class membership, property values,
and (in)equality of individuals.
OWL DL is defined in terms of an abstract syntax [109]. However, the
normative syntax for OWL is RDF/XML. The RDF representation of an
OWL DL ontology can be obtained through a mapping from the abstract
syntax. Listing 3.1 shows an OWL DL ontology, in abstract syntax, where
we define a class Person with a property hasChild,oftypePerson,andaclass
Parent, which is defined as a person who has a child. Finally, we define an
individual John, who has a child Mary. OWL DL allows us to infer that John
is a Parent.
Note that from the type restriction on hasChild in the definition of Person,
and the fact that Mary is a value for this property, Mary is in fact a member of