Nonmonotonic reasoning

The research on nonmonotonic reasoning includes several attempts to formalize reasoning that refuse to acknowledge one of the fundamental properties of classical logic: monotonicity. Nonmonotonic formalizations of reasoning deal with the problem of drawing conclusions when the description of either a situation or a problem is incomplete. Such incompleteness can be justified not only by epistemological reasons, since the knowledge about a situation can often be partial, but also by engineering reasons, as the need arises to avoid the explicit recording of a huge amount of information. In this paper we describe the main formalizations of nonmonotonic reasoning with the aim to present a coherent overview of the research in this field, and therefore we pay special attention to the problem of establishing relationships among different proposals.