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turns out that rules languages cannot be layered on top of the Web Ontology
Language OWL in a straightforward manner [70]; this triggered a refinement
of the layer cake, depicted in Fig. 3.1, where rules feature next to OWL, on top
of a common layer. The revised layer cake
2
was presented by Tim Berners-Lee
at his keynote address at the WWW2005 conference.
Fig. 3.1.
The Semantic Web language layer cake
The bottom layers in the layer cake, i.e. Unicode and URI and XML
(Schema), consist of existing Web standards and provide a syntactical basis for
Semantic Web languages. Unicode provides an elementary character-encoding
scheme, which is used by XML. The URI (uniform resource identifier) stan-
dard provides a means to uniquely identify and address documents and, more
generally, resources on the Web. All concepts used in the languages located
higher in the layer cake can be specified using Unicode and are uniquely iden-
tified by URIs.
We shall describe the RDF(S), OWL, and rules layers below. We shall not
cover the logic, proof, and trust layers here. Placing the logic layer on top
of the OWL and rules layer is somewhat controversial, since OWL and rules
languages are grounded in logic. Some argue that a more expressive logic
language should be layered on top of the ontology language [108]. It could
also be argued that this is not an appropriate layering; that is, that OWL and
rules should be the top languages and that applications should use that layer
directly. The proof and trust layers are not well-understood, but most likely
refer to the application and not to any specific language. For instance, the
application could prove some statement by using deductive reasoning, and
a statement could be trusted if it had been proven and digitally signed by
some trusted third party. The user would very likely play an important role
in the trust layer because it is the user that should decide whether or not an
information source should be trusted.
2
http://www.w3.org/2005/Talks/0511-keynote-tbl/
.
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