A Logic for Default Reasoning


The need to make default assumptions is frequently encountered in reasoning'about incompletely specified worlds. Inferences sanctioned by default are best viewed as beliefs which may well be modified or rejected by subsequent observations. It is this property which leads to the non.monotonJcity of any logic of defaults. In this paper we propose a logic for default reasoning. We then specialize our treatment to a very large class of commonly occurring defaults. For this class we develop a complete proof theory and show how to interface it with a top down resolution theorem prover. Finally, we provide criteria under which the revision of derived beliefs must be effected. The gods did not reveal, from the beginning, All things to us, but in the course of time Through seeking we may learn and know things better. But as for certain truth, no man has known it, Nor shall he know it, neither of the gods Nor yet of all the things of which I speak. For even if by chance he were to utter The final truth, he would himself not know it: For all is but a woven web of guesses. Xenophanes

DOI: 10.1016/0004-3702(80)90014-4

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@article{Reiter1980ALF, title={A Logic for Default Reasoning}, author={Raymond Reiter}, journal={Artif. Intell.}, year={1980}, volume={13}, pages={81-132} }