Aristotle, Rhetoric and Probability Katarzyna Budzynska 1 , Magdalena Kacprzak 2 1 Institute of Philosophy, Cardinal Stefan Wyszynski University in Warsaw, Poland k.budzynska@uksw.edu.pl 2 Faculty of Computer Science, Bialystok University of Technology, Poland mkacprzak@ii.pb.bialystok.pl Abstract. The aim of the paper is to explore and link three concepts together – the role of probability in rhetoric, the meaning of this notion according to Aristotle’s theory of persuasion and the formal tools for representing the probability in important in rhetoric interpretations. We focus, especially, on applications of objective and subjective probability. For their formalization we adapt logics introduced by J. Y. Halpern. Introduction In the paper we are interested in exploring the following research questions: What role plays a notion of probability in rhetorical studies? Exists only one way of referring to this notion during the persuasion? For example, does an expression “Probably a person who commits a crime leaves trace evidence” use the notion of probability in the same meaning as an expression “John is probably guilty”? What Aristotle states about the probability? What formal tools should we use to represent the interpretation that is important for rhetoric? We examine two levels of applying the notion of the probability in rhetoric. The first level pertains to a frequency of satisfaction of an argumentation’s premise that expresses a statistical relationship. It uses the objective interpretation of the probability like e.g. in the statement: “Probably a person who commits a crime leaves trace evidence”. It suggests a statistical relationship between committing a crime and leaving trace evidence, i.e., when the first phenomenon takes place then probably (almost always, usually) the other will take place as well. The second level of application uses the subjective interpretation of probability which is related to the power of arguments or the strength of belief in a thesis. In other words, it is the measure of the uncertainty of persuasion’s participants. For example, the probability used in the expression “John is probably guilty” refers to the uncertainty about John’s guilt. The key difference between these two types of probabilities is the fact that statements of objective probability refers to real, statistical state of the world, while statements of subjective probability refers to someone’s beliefs about the world. It is hard to not notice that the notion of probability plays an important role in Aristotle’s rhetoric. Originally he uses the objective interpretation of probability referring to the fact that people more often persuade each other about what happens usually than about what happens always. However, the subjective interpretation of probability can be also found