Not even scientific texts are mere subject matter, but follow general rules of rhetoric . Rhetoric is in itself a theory of knowledge. The rhetorical approach includes the existence of the logical approach. The logical demonstration, however, denies the need for the rhetorical theory of knowledge. You may find a similar relation between positivism and hermeneutics.
Positivism excludes while hermeneutics includes.
Because Perelman looks to a juridical model of truth rather than a mathematical one, he does not assume, as does Descartes, that only one side can be right in a dispute. Perhaps both are right, perhaps both views could be proven in the process of dialogue and argumentation, and finally, as practical solutions. In many ways the framework of argumentation is the most important factor,I.E. defining the relationship to the audience. You have to adjust your argumentation in relation to an audience. The audience is always a key factor in all rhetoric.