Abstract Argumentation is considered here as discourse that involves arguments, defined as
reason (s)—conclusion pairs. In an argument, reasons are oriented in favor of the
conclusion. It can happen that an argument is used to argue against the conclusion of
another argument. This article is devoted to this negative aspect of argumentation, which
has seldom been studied elsewhere. After insisting on the dialogical dimension of every
argumentation, the hypothesis is developed that an argument is accepted only if the reasons …