Description logic
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Description logics are a family of knowledge representation languages which can be used to represent the terminological knowledge of an application domain in a structured and formally well-understood way. The name description logic refers, on the one hand, to concept descriptions used to describe a domain and, on the other hand to the logic-based semantics which can be given by a translation into first-order predicate logic. Description logic was designed as an extension to frames and semantic networks, which were not equipped with a formal logic-based semantics.
Description logic was given its current name in the 1980s. Previous to this it was called (chronologically): terminological systems, and concept languages. Today description logic has become a cornerstone of the Semantic Web for its use in the design of ontologies.
The first DL-based system was KL-ONE (by Brachman and Schmolze, 1985). Some other DL systems came later. They are LOOM (1987), BACK (1988), KRIS (1991), CLASSIC (1991), FaCT (1998) and lately RACER (2001), CEL (2005), and KAON 2 (2005).
The development of OIL was inspired by DL.
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Syntax and Semantics of Description Logics
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Modelling in Description Logics
In DLs, a distinction is drawn between the so-called "TBox" (terminological box) and the "ABox" (assertional box). In general, the TBox contains sentences describing concept hierarchies (i.e., relations between concepts) while the ABox contains "ground" sentences stating where in the hierarchy individuals belong (i.e., relations between individuals and concepts). For example, the statement:
(1) Every employee is a person
belongs in the TBox, while the statement:
(2) Bob is an employee
belongs in the ABox. Note that the TBox/ABox distinction is not significant in the same sense that in first-order logic (which subsumes most DLs). The two "kinds" of sentences are not treated differently. When translated into first-order logic, a subsumption axiom like (1) is simply a conditional restricted to unary predicates (concepts) with only variables appearing in it. Clearly, a sentence of this form is not privileged or special over sentences in which only constants ("grounded" values) appear like (2).
So why was the distinction introduced? The primary reason is that the separation can be useful when describing and formulating decision-procedures for various DLs. For example, a reasoner might process the TBox and ABox separately, in part because certain key inference problems are tied to one but not the other one ('classification' is related to the TBox, 'instance checking' to the ABox). Another example is that the complexity of the TBox can greatly affect the performance of a given decision-procedure for a certain DL, independently of the ABox. Thus, it is useful to have a way to talk about that specific part of the KB.
The secondary reason is that the distinction can make sense from the knowledge base modeller's perspective. It is plausible to distinguish between our conception of terms/concepts in the world (class axioms in the TBox) and particular manifestations of those terms/concepts (instance assertions in the ABox.)
Differences with OWL
Terminology
A concept in DL jargon is referred to as a class in OWL. A role in DL jargon is a property in OWL.
Names
Should add discussion of unique names assumption (UNA) versus no unique name assumption. OWL does not make the UNA.
Description Logic Reasoners
There are some reasoners to deal with OWL and Description Logics. These are some of the most popular:
- CEL is a free (for non-commercial use) LISP-based reasoner
- Cerebra Engine is a commercial C++-based reasoner.
- FaCT++ is a free open-source C++-based reasoner.
- KAON2 is a free (free for non-commercial usage) Java reasoner.
- MSPASS is a free open-source C reasoner for numerous description logics.
- Pellet is a free open-source Java-based reasoner.
- RacerPro is a commercial (free trials and research licenses are available) lisp-based reasoner
Other tools related to Description Logics include the following:
- Protégé is a free, open source ontology editor and knowledge-base framework, which can use DL reasoners which offer a DIG interface as backends for consistency checks.
See also
References
- F. Baader, D. Calvanese, D. L. McGuiness, D. Nardi, P. F. Patel-Schneider: The Description Logic Handbook: Theory, Implementation, Applications. Cambridge University Press, Cambridge, UK, 2003. ISBN 0-521-78176-0