Rules, logic and Inference
Overview
The architecture of the Semantic Web is comprised of a series of layers
that form a hierarchy of content and logic. The ontology layer defines knowledge
about content, concepts, and relationships. Currently, the RDF Schema (RDFS) is
recognized as an ontology language that provides classes, properties,
sub/super-classes, range and domain. However, RDFS has no localized range and
domain constraints, no cardinality constraints, no provision for negation, and
no transitive, inverse or symmetrical properties. As a result, RDFS is unable to
provide sufficient expressive power for machine processing on the Semantic Web.
To expand the expressive capabilities of RDFS, three versions of the Web
Ontology Language (OWL) have been developed: OWL Full is the union of OWL syntax
and RDF, but it is undecidable and therefore can not provide complete reasoning
support. OWL DL is a sublanguage of OWL Full that has efficient reasoning
support, but is not fully compatible with RDF. OWL Lite is an ‘easier to
implement’ subset of OWL DL.
RDF and OWL DL are specializations of predicate
logic (also known as first order logic (FOL)) that are used for Web knowledge
representation. They provide a syntax that promotes their use on the Web in the
form of tags. OWL DL and OWL Lite correspond roughly to a descriptive logic
which is a subset of predicate logic for which there exists adequate proof
systems. Another subset of predicate logic with efficient proof systems is the
rule system; Horn Logic.
The choice of OWL version and complementary rule
systems will dictate the resulting computational complexity of the Semantic Web.