This dissertation discusses the rhetorical aspects of mathematical theory about the probable from a historical perspective. While today the term "probability" is largely associated with mathematics, this is historically a recent development; before the mid-seventeenth century, the term was used extensively in forensic rhetoric but had no mathematical definition at all. I begin this project by discussing the uses of this term as a specifically rhetorical term through the history of mathematics up through the seventeenth century. I continue by then discussing probability mathematics and its history, focusing on the earliest works of true probability mathematics, the mid-seventeenth century The Art of Thinking and Jakob Bernoulli's early eighteenth-century Ars Conjectandi. My contention is that handbooks of forensic rhetoric (such as the Ad Herennium, De Inventione, and parts of Quintilian's Institutio Oratoria) acted as an important influence on very early works of probability mathematics, and I closely examine the relevant sections of The Art of Thinking and the Ars Conjectandi to show the similarities of these works to rhetorical discussions of probability. In the final chapter of this work, I discuss four issues: first, I discuss the changes in rhetorical arguments about the probable since the end of the seventeenth century. Second, I discuss the effects of such changes on rhetoric as a subject: given that argumentation about the probability of events once constituted a large and well-structured part of rhetorical training but is no longer treated so thoroughly, what does this mean about the uses of rhetorical and informal argument? What role does this change in the term "probability" play in the history of rhetoric? Third, I look at mathematical probability as a tool of argument today with reference to the above-mentioned historical investigation: how are the roots of that branch of mathematics relevant to its practice today? That is to say, how is modern probability mathematics employed in public discourse today? Fourth and finally, how does such a study fit into other, similar research that seeks both to expand the history of rhetoric and to examine the rhetorical nature of self-proclaimed arhetorical discourse?