A non-monotonic logic is a formal calculus where the consequence relation is not monotonic. Intuitively, nonmonotonicity of the consequence relation indicates that obtaining a new piece of information can reduce the set of the accepted sentences. In particular, it allows us to draw conclusions on the basis of 'the lack of evidence to the contrary'. The purpose of the paper is to present the basic notions of non-monotonic logic, which will be needed in two papers in this volume.
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