Aristotle's Rhetoric on Unintentionally Hitting the Principles of the Sciences
“Rhetoric cannot be given a definition by genus and differentia, and so Aristotle must rely on the contrasts between rhetoric and its neighbors to an extent unnecessary for the definition of many of the arts and sciences. In addition to its positive definition as a faculty of finding in any case the existing means of persuasion he marks off its boundaries through a series of contrasts to other ways of achieving the same end – persuasion – that rely on sources of persuasion outside of discourse and hence outside the art of rhetoric. In the first chapter rhetoric is restrained by good laws that prohibit irrelevant appeals; in the last chapter of Book I Aristotle contrasts its practice with the inartificial means of persuasion, such as written records, that only have to be used; in the passages I want to concentrate on here, rhetoric is bounded by the sciences. We learn about this latter boundary by looking at its violations: when a speaker or thinker unconsciously and unintentionally "hits on a principle," Aristotle tells us that he thereby moves from rhetorical to scientific argument.”
Garver, Eugene. "Aristotle's Rhetoric on Unintentionally Hitting the Principles of the Sciences." Rhetorica 6, no. 4 (Autumn 1988): 381-393. doi:10.1525/rh.19220.127.116.111.