Since Antiquity dialogue was one of the paradigmatic genres in theoretical discourse and its cognitive function was of fundamental significance in Renaissance theory of dialogue. This importance was considered for the first time by Carlo Sigonio in his De dialogo liber. Referring to Aristotle’s Topica, Sigonio defined dialogue as a written imitation of a dialectical disputation. He considered the arguments in a dialogue as well as in a dialectical disputation not to be based on scientific propositions, which are necessarily true, but to be based on commonly accepted premises. In consequence thereof, dialogue is not intended to generate indubitable knowledge but to foster „opinions“ (opinio), „beliefs“ and „convictions“ (fides), whose epistemological value is a matter of discussion. The intention of this paper is to present the arguments developed by Sigonio in order to explain that opinio and fides are not only legitimate epistemic goals, but sometimes the only reasonable ones. A further topic is the functions of dialectic and rhetoric within the framework of this cognitive concept.