In this paper we present a novel approach to default reasoning, based on the concept of A-uncertainty, namely uncertainty concerning rule applicability. The paper first presents a focused analysis, based on some simple default reasoning examples, of the limitations of some well-known approaches to default reasoning. Then, the novel approach, based on the explicit representation of A-uncertainty is introduced. It is based on a concept of A-uncertainty intended as a property concerning both an inference rule and the individual to which it is applied, and it is shown to be appropriate to overcome most of the limitations found in classical approaches in a natural and effective way. A general default reasoning scheme is then proposed, which exploits the advantages of the introduced representation and sets up a promising background for efficient implementations of automated reasoning.
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