4.4 The Web Ontology Language 79
- The RDF languages have a formal semantics. XTM only has a formal
metamodel.
- XTM makes a clear distinction between metadata and data, while RDF
does not. In RDF one can annotate anything, including annotations.
4.4 The Web Ontology Language
We began this chapter with a story about the experience people sometimes
have of suddenly realizing that two of their acquaintances are actually the
same person. Although RDF introduces some important prerequisites for
reasoning about whether two entities are the same or different, it does not
have the ability to deal with this issue. OWL differs from RDF in a number
of ways, but one of the most important is how it deals with identity. One
can explicitly state that two resources are the same or that they are different.
It is also possible to infer one or the other of these two cases. However, it
is also possible that one will not be able to infer either one. This has a pro-
found impact on the semantics of OWL. Unlike XML or RDF, one must now
consider many “possible worlds,” differing from one another with respect to
whether resources are the same or different, yet each of the possible worlds
is compatible with the known facts.
Logicians refer to the process of equating entities asparamodulation.The
OWL paramodulation properties aresameAsanddifferentFrom.Inad-
dition to paramodulation, OWL extends the RDF vocabulary to include rela-
tions between classes (such as disjointness), cardinality constraints on prop-
erties (e.g., "exactly one"), characteristics of properties (e.g., symmetry), and
enumerated classes. All of these features of OWL are intimately connected to
the issue of identity. For example, suppose that one states in the molecule on-
tology that everyatomhas exactly oneelementType. If a particular atom
does not have an elementType, then one can infer that this atom has an el-
ementType which is thesameAsone of the known element types, but one
does not know which one. No inference of this kind is possible in RDF.
The OWL language specification is given in (van Harmelen et al. 2003).
There are three distinct OWL languages: OWL Lite, OWL-DL, and OWL Full.
They differ from each other primarily with respect to what constructs are
allowed in each language. OWL Lite is the most restrictive. OWL-DL is less
restrictive than OWL Lite but more restrictive than OWL Full. OWL Full is
unrestricted. Syntactically, OWL is nearly identical to RDF. Like RDF, one can
intermix ontological and data statements. The only syntactic differences are
