Wilson, Douglas, and Nathan D. Wilson. The Rhetoric Companion: A Student's Guide to Power in Persuasion. Moscow, Idaho: Canon, 2011.
Lesson 2, “The Purposes of Rhetoric” pp. 17-21
Wilson summarizes his view of the purpose of rhetoric. “The point of true rhetoric, in all its guises, is to deal with ignorance, bring about like-mindedness, and motivate to action” (Wilson 2011, 17). He compares this to the definition by Aristotle (Rhetoric, I.2) which views rhetoric as discovery of persuasive means. Aristotle’s definition may be inadequate, compared with Quintilians’s definition, cited on p. 14, involving being good and speaking well. Wilson then introduces us to three rhetorical categories classified in antiquity.
The judicial use of rhetoric is that which would be used in a court of law. In Roman society the actual arguments were of great importance, though they are not of as much importance in the United States now.
The deliberative use of rhetoric is to consider a course of action. It asks questions, typically in order to determine the best course of action.
The epideictic use of rhetoric uses praise or blame to demonstrate whether someone or something should be considered good or bad.
Wilson considers on p. 19 that the Christian sermon is a further category of rhetoric, one which has spread into different areas of life, such as political speeches. The use of rhetoric to bind or loose a conscience is fairly common in our day. Wilson suggests that preaching is divided into two realms, that of credenda - that which should be believed and that of agenda - that which should be done (Ibid., 19).