Recent criticism on the Renaissance has drawn attention to an interest in the sixteenth century in the rhetorical methods of argumentation which were used to support, and lend legitimation to, new positions or ideas. Such methods of 'proof', which were designed to create belief, replaced the logical methods of argumentation of the medieval scholastics, which were designed instead to discover the true principles of a particular line of enquiry. I explore a different and complementary method for the creation of credibility, a method which derives from the same Roman rhetorical tradition, and which was promoted by Renaissance writers including Philip Sidney. This method originates with Marcus Tullius Cicero, although Cicero himself suggests that he is inspired by the Socrates of Plato's dialogues. The claim of Plato's Socrates not to know the rhetorical techniques for producing eloquence, a claim which is meant to indicate that his speech is prompted by a genuine knowledge of true principles, is interpreted by Cicero itself as a rhetorical strategy for creating an ironic persona which conceals the studied nature of his expression and which thus gains him in the credence, and trust, of his audience. In his rhetorical manuals Cicero imitates, and thus demonstrates, this supposed ironic strategy, for two seemingly contradictory reasons: on the one hand, he wishes to elevate the status of the art of rhetoric - the art held in contempt by Plato - showing that it is a legitimate and fundamental tool for intellectual enquiry; on the other hand, he wants to conceal the studied nature of his own eloquence so that his audience believe that he speaks as nature or truth prompts him.