Abstract
The distinction between constitutive and regulative rules is applied to rules for critical discussion that have to do with the use of ambiguous expressions. This leads to a distinction between rule violating fallacies, by which one abandons a critical discussion, and norm violating fallacies, which are in a way admissible within a critical discussion. According to the formal model for critical discussion, proposed in this paper, fallacies of the norm violating type arc not prohibited. Instead, it provides discussants with devices to discuss fallacies and fallacy criticisms