We define an inference system to capture explanations based on causal statements, using an ontology in the form of an IS-A hierarchy. We introduce a simple logical language which makes it possible to express that a fact causes another fact and that a fact explains another fact. We present a set of formal inference patterns from causal statements to explanation statements. We introduce an elementary ontology which gives greater expressiveness to the system while staying close to propositional reasoning. We provide an inference system that captures the patterns discussed, in a datalog (limited predicate) framework.

Recently, the old logical notion of forgetting propositional symbols (or reducing the logical vocabulary) has been generalized to a new notion: forgetting literals. The aim was to help the automatic computation of various formalisms which are currently used in knowledge representation. We extend here this notion, by allowing propositional symbols to vary while forgetting literals. We describe the new notion, on the syntactical and the semantical side. We provide various different syntactical characterizations, in order to provide various methods for computing the notion introduced here. This confirms that one of the main interests of the notion of ‘forgetting literals’ (original, and new version) is that it provides new kinds of methods of computation. Then, we show how to apply it to the computation of circumscription. This computation has been done before with standard literal forgetting, but here we show how introducing varying propositional symbols simplifies significantly the computation. We revisit a fifteen years old result about computing circumscription, showing that it can be improved in the same way. We provide hints in order to apply this forgetting method also to other logical formalisms.

Theories of change and nonmonotonic formalisms originated from the same knowledge representation problems. the differences in their respective aims and methods are illustrated by a comparison between the possible model approach and preferential entailment. Then, one technical way of using a nonmonotonic formalism in order to achieve cognitive change is shortly described, and the same for the other way around. the more philosophically oriented two way passage by Gärdenfors and Makinson is then examined at length, including the newest versions providing a general translation through epistemic entrenchment. Some kind of “concretization” of these theoretical considerations are then described. Finally, thanks to Gärdenfors and Makinson's passage, we translate some important criteria for belief revision into nonmonotony. This general comparison provides as many insights on the nonmonotonic side as on the cognitive change side. © 1994 John Wiley & Sons, Inc.