Default and Inexact Reasoning with Possibility Degrees

A new approach, which is based on possibility theory, for dealing with uncertain facts and default rules is proposed. The chaining of certain or uncertain facts and rules is discussed in detail. The modus ponens, themodus tollens, and the resolution principle in the propositional case are generalized and patterns of plausible reasoning are recovered. Only min and max operations are used for computing the possibility degrees corresponding to the different alternatives. The problem of the combination of results obtained from differentsources is discussed. It is shown that the combination may be completely unsuitable when some uncertain conclusions can be obtained directly through a specific rule and inidirectly through a chain of inferences. The observation has important consequences when building inference systems for the exploitation of uncertain knowledge bases.