The Semantic Web - Social Semantics: The Search for Meaning on the Web - page 62

so merged with the triple itself. An RDF(S)-aware agent that has retrieved the
RDF Schema can deduce from the triple that ex:Gustave Eiffel rdf:type
ex:Person , namely that Gustave Eiffel is indeed a person. This sort of simple
reasoning is again encoded as a set of axiomatic triples and rules for inference and
semantic conditions for applying these axioms to infer more triples. See the RDF
Formal Semantics for full details (Hayes 2004). From here on out, the acronym
'RDF' refers to both RDF and RDF(S), whose formal semantics are given together
(Hayes 2004).
In practice, the Principle of the Open World has surprising results. One of the
ramifications in RDF is that there is no proper notion of false, but only the notion
that something is either inferred or not, and if it is not inferred, it may simply be
undefined. Although it seems straightforward, in practice this leads to surprising
results. Take the following example: 'Gustave is the father of Valentine,' which in
RDF is ex:Gustave ex:fatherOf ex:Valentine .IsGeorgealsothe
father of Valentine ( ex:George ex:fatherOf ex:Valentine )? Operating
under the closed world assumption, the answer would be no. Yet operating under
the Open World Principle, that statement would be possible, for there is no
restriction that someone can only have a single father, and in RDF(S) stating
such a restriction is impossible. This restriction is possible in the Web Ontology
Language (abbreviated OWL, in an obscure reference to A.A. Milne), an open-
world extension of RDF that allows restrictions, such as cardinality, to be placed
on predicates. However, even if one sets the cardinality of the ex:fatherOf
predicate to one (so that a person could have at most one father), the results would
be surprising: the reasoner would conclude that ex:George and ex:Gustave
refer to the same individual. In contrast to the expected behaviour of many other
inference engines, including people, there is no Unique Name Assumption , the
assumption that each unique name refers to a unique individual , due to the Open
World Principle. The Unique Name Assumption, while very useful for counting,
makes an implicit assumption about each name referring to only one individual, and
if an individual cannot be found that satisfies the name then that individual must not
exist. This further reinforces the tendency of URIs on the Semantic Web, despite
their global scope, to be ambiguous, a point we shall return to.

3.2.5 RDF and the Principle of Least Power

Insofar as it is applied to the Semantic Web, the Principle of Least Power is
strangely counter-intuitive: traditionally knowledge representation languages were
always striving for greater power, yet the Semantic Web begins with RDF, a
language purposefully designed to be the least powerful language. The true bet of
the Semantic Web is then on triples as the most basic language upon which other
languages can be based. The challenge for the Principle of Least Power is how to
build the rest of the Semantic Web by expanding on the language of triples.