proposed changes to Document Overview

[I don't think that these are necessary for the FBWD publication, but it
would be nice to get them in.]


=== Semantics ===

The OWL 2 Structural Specification document defines the abstract
structure of OWL 2 ontologies, but it does not define their meaning.
The Direct Semantics [<cite>[[#ref-dlsem|OWL 2 Direct
Semantics]]</cite>] and the RDF-Based Semantics
[<cite>[[#ref-owl-2-??|OWL 2 RDF-Based Semantics]]</cite>]
provide two alternative ways of assigning meaning to OWL 2 ontologies,
with a correspondence theorem providing a link between the two.  These
two semantics are used by reasoners and other tools to answer queries
about, e.g., class consistency, subsumption and instance retrieval.

The Direct Semantics assigns meaning directly to ontology structures,
resulting in a semantics compatible with the model theoretic semantics
of the ''SROIQ'' description logic&mdash;a fragment of first order logic
with useful computational properties. The advantage of this close
connection is that the extensive description logic literature and
implementation experience can be directly exploited by OWL 2
tools. However, some conditions must be placed on ontology
structures in order to ensure that they can be translated into a
''SROIQ'' knowledge base, for example, transitive properties cannot be
used in number restrictions (see [[Syntax#Ontologies|Section 3]] of the
OWL 2 Structural Specification document
[<cite>[[#ref-owl-2-specification|OWL 2 Specification]]</cite>] for a
complete list of these conditions).  Ontologies that satisfy these
syntactic conditions are called “OWL 2 DL” ontologies.

The RDF-Based Semantics assigns meaning directly to RDF graphs and so
indirectly to ontology structures via the Mapping to RDF-graphs. The
RDF-Based Semantics is fully compatible with RDF Semantics
[<cite>[[#ref-rdfsem|RDF Semantics]]</cite>], and extends the semantic
conditions defined for RDF.  The RDF-Based Semantics can be applied to
''any'' OWL 2 Ontology, without restrictions, as any OWL 2 Ontology can
be mapped to RDF.<sup>2</sup> This semantics for OWL 2 Ontologies is
sometimes called the “OWL 2 Full” semantics and “OWL 2 Full” is also
used to refer to the entire OWL 2 language, particularly when expressed
as RDF graphs.

The correspondence theorem in Section 7.3 of the RDF-Based Semantics
Document [<cite>[[#ref-??|OWL 2 RDF-Based Semantics]]</cite>]) defines a
precise, close relationship between the Direct and RDF-Based
Semantics. This theorem states, in essence, that given an OWL 2 DL
ontology, inferences drawn using the Direct Semantics will still be
valid if the ontology is mapped into an RDF graph and interpreted using
the RDF-Based Semantics.

Received on Wednesday, 11 March 2009 00:21:08 UTC