Abduction is not Deduction-in-Reverse

Abduction is a topic that attracts much interest in AI and automated reasoning research. Diierent approaches have been devised, that give a formalized account of explanatory reasoning, propose methods to compute explanations, frame abduction in the context of logic programming. However, the logical nature of abduction is still far from being clear and diierent speciications of the key underlying concepts have been given, that make it diicult to speak of abduction as a single well-deened form of reasoning. This work is a preliminary discussion on the logical nature of abductive reasoning, emphasizing the fundamental diierence between abductive and deductive inference. Some logical properties of the inference to the \best explanation" are put forward and analyzed when the underlying logic is any extension of classical propositional logic ((rst order logic, modal logic) or a non monotonic system.