# Default Logic: From Theory to Applications

### Ilkka Niemelä

A course given at
the 11th European Summer School in Logic, Language and Information
,
Utrecht University, Aug 16-20, 1999.
### Course Description

Nonmonotonic reasoning has been one of the most active research areas in
knowledge representation and reasoning since the late seventies and
Reiter's default logic has turned out to be one of the most successful
nonmonotonic logics. Default logic offers a very expressive framework
for handling knowledge representation tasks in a compact, robust and
flexible way even in the presence of difficult issues like the frame
problem and the qualification problem. However, the development of
applications based on default logic has not been possible because of the
lack of efficient implementations. Recently, the situation has changed
by breakthroughs in computational techniques for default reasoning
leading to efficient implementations for fragments of Reiter's default
logic. For example, the Smodels system, which is an implementation of a
subset corresponding to normal logic programs with the stable model
semantics, is able to handle realistic size problems (tens of thousands
of default rules) with competitive performance. For instance, in
planning it has shown comparable and occasionally even better
performance than the best domain independent planning systems such as
Graphplan.
The objective of the course is to introduce basic default logic and its
most important subclasses corresponding to logic programs, explain
relevant expressivity and complexity results, present implementation
methods behind the computational breakthrough and discuss application
methodology for default reasoning. An interesting approach to developing
applications for default reasoning is to see default logic as a
framework for declarative rule-based constraint programming. This
approach is introduced, its relation to standard constraint satisfaction
is explained and examples of applications in combinatorial problems,
planning, and product configuration are discussed. A distinguishing
feature of the course is the availability of an efficient
implementation, the Smodels system, which makes it possible to obtain
hands-on experience in using default reasoning techniques.

### Slides for the lectures:

### Material