Reasoning with incomplete information constitutes a major challenge for any intelligent system. In fact, we expect from such systems that they are not paralyzed by missing information but rather arrive at plausible results by bridging the gaps in the available information. There are moreover situations in which it is necessary to act despite incomplete information. In a word, intelligent systems have to provide means for reasoning in the absence of total information. The automation of this form of reasoning is the subject of this book.
A versatile way of reasoning in the absence of information is to reason by default. In this manner, we overcome a lack of information by making default assumptions about a situation at hand. Default reasoning puts faith in standard situations that are free from strangeness and unexpectedness; it relies on general rules expressing anticipated states of affairs. A formalization of this form of reasoning is furnished by default logic - a logical system developed by Raymond Reiter in [Rei80].
This book aims at providing formal and practical means for automating reasoning with incomplete information by starting from the approach taken by default logic. For this endeavor, we span a bridge between formal semantics, over logical systems for default reasoning, to efficient implementations.