4.2 The Resource Description Framework 73
The fundamental relationship between classes in a taxonomy is thesubclass
relationship. For example, in the chemical hierarchy in figure 1.11, Macro-
molecule is a subclass of Chemical, and Protein is a subclass of Macromolecule.
In RDF, the subclass relationship is calledrdfs:subClassOf. To specify
that Protein is a subclass of Macromolecule, which is a subclass of Chemical,
one would use these RDF statements:
Now suppose that one is describing a particular protein using RDF:
RDF will automatically infer that rhodopsin is also a macromolecule and a
chemical. In other words, the fact that rhodopsin is a macromolecule is inher-
ited from the fact that it is a protein. This can be important for information
retrieval and information transformation. Without this inference, a query for
a chemical would not recognize rhodopsin as being a chemical.
As discussed in chapter 3, a rule has an antecedent and a consequent. The
antecedents for RDF rules consist of one or more RDF statements. The con-
clusion consists of exactly one RDF statement. If the antecedent statements
have been previously asserted (i.e., either explicitly stated or previously in-
ferred), then the consequent statement is inferred. The most important infer-
ence rules that are built into RDF are the following:
- Subclass rule. If classAis asubClassOfclassB, and if the resource
Rhasrdf:typeA, then the resourceRalso hasrdf:typeB. In other
words, classAis a subset of classB. - Subproperty rule.If propertyPis asubPropertyOfpropertyQ,andif
the resourceRis linked to resourceSusing propertyP, then the resource
Ris also linked to resourceSusing propertyQ. In other words, the links
(statements) using propertyPare a subset of the links usingQ. - Domain rule.If propertyPhasdomainD, and if the resourceRis linked
to resourceSusing propertyP, then the resourceRhasrdf:typeD.