In this article we propose a methodological reflection of philosophical work, based on a study of the confused notions raised by Chaïm Perelman in his theory of argumentation, also called the new rhetoric. In this way, we work two times in which Perelman addresses the problem of confused notions. A first moment in which, under positivist assumptions, Perelman seeks to submit confused notions of ordinary language to the schemes of formal logic, being Justice the core text of our analysis. And a second time when, from the claims of rhetoric since 1952 and also the recognition of the dynamism of the confused notions in particular contexts of argumentation, Perelman argues the methodological status of discourse and demonstrative evidence, i.e. ...