An argument can be taken as an operation of justification or as the product of this operation. But what about a counter-argument? This article is based on the hypothesis that there exists an operation of argumentative negation, which is both the argumentative and the negative equivalent of the operation of justification. Justification and argumentative negation necessarily act on assertions, for they are active at the level of the epistemic modalities of statements. As an operation, a counter-argument can thus be described, as the application of an argumentative negation; as a product, it can be described as an argument the conclusion of which shows traces of negative modality. Two uses of argumentative negation are distinguished here: counter-argumentation and calling into question. The former can lead only to assertive conclusive statements, but the latter can also lead to directives or commissives. This leads the authors to introduce the notion of pseudo-argument, besides those of argument and counter-argument. It is shown in particular that when a pseudo-argument is rejected, argumentative negation has the effect of making evident an underlying argument. With respect to the latter it functions as a counter-argumentation, whereas with respect to the illocutionary act accomplished by the conclusive statement, it functions as a calling into question of a condition of satisfaction for this act. This article also defines certain characteristics of argument, proposes criteria for identifying argumentative negation in polemical conversations, and distinguishes four modes of counter-argumentation. RESUME: Un argument peut etre appr6hend6 en tant qu'op6ration de justification ou en tant que produit de cette operation. Mais qu'en est-il d'un contre-argument? Cet article est fond sur l'hypothese qu'il existe une operation de negation argumentative, equivalent a la fois argumentatif et ngatif de l'operation de justification. La justification et la negation argumentative portent ncessairement sur des assertions, car elles agissent au niveau des modalit6s pistdmiques des nonc6s. Un contre-argument peut alors tre d6crit, en tant qu'operation, comme I'application d'une ngation argumentative, et, en tant que produit, comme un argument dont la conclusion porte la trace d'une modality negative. Deux categories d'usages de la negation argumentative sont ici distingu6es: la contre-argumentation et la mise en cause. Si la premiere ne peut enchainer que sur des enonces conclusifs assertifs, la seconde peut aussi enchainer sur des directifs ou des