Aristotle's enthymeme and nonmonotonic logics

In this paper I explore the usefulness of nonmonotonic logics within the domain of enthymematic reasoning. Nonomonotonic logics are logical systems that do not require certain and complete information and allow for fallibilistic or merely plausible reasoning. In order to bring the work of contemporary nonmonotonic logical theory into the realm of rhetoric, I provide a brief sketch of the history of logic in the twentieth century and argue that the hostility between rhetoricians and logicians is in part a result of broad misunderstanding. An understanding of contemporary rather than a Fregean, turn-of-the-twentieth-century metatheory could provide the rhetorician with a useful tool for explaining different reasoning patterns in the overlap between logical domains and rhetorical situations. After sketching the relationship between nonmonotonic and classical logics, I argue that nonmonotonic logics could be useful for modeling the type of reasoning Aristotle presents with his conception of the enthymeme. To show the rhetorical function of nonmonotonic logics, I examine plausibilistic reasoning in two texts: an excerpt on diagnosing chronic cystic disease from L.V. Ackerman's 1970 textbook Cancer and an early speech on university education from John Stuart Mill. These seemingly incongruent examples are not provided in an effort to show that nonmonotonic logics can account for all instances of rhetorical reasoning but are instead provided to show an understanding of logical domains can be useful in radically dissimilar rhetorical situations.