Interactive Exercise

What is evidence?
Evidence is the supporting material you use to prove a point. Doing research and applying standards of critical thinking to evaluate evidence are important responsibilities of a public speaker. Doing research was also an important part of the classical art of rhetoric, which Aristotle defined as the "art of discovering the available means of persuasion in a given situation."

What types of evidence should I use in my speech?
The answer to this question is, 'Well...it depends." It depends on the basic elements of your speaking situation:

• What is your experience and expertise on the topic?
• Who is the audience?
• What is the speaking occasion?
• What is your speaking goal and the genre of discourse?

Experience and expertise establish your credibility to speak about the topic. We all have a body of personal knowledge made up of all of our experiences; this knowledge can be the starting point for your speech. If you can qualify yourself as an expert--in the mind of the audience--the evidence you use outside your own direct experience will simply amplify the points you are making. Lacking that audience perception of your expertise, you will need to use evidence to prove your points.

The audience will also dictate how much and what type of evidence to use. As a general rule, you will need to use more evidence if the audience is more skeptical about your topic, or if the audience questions your credibility. Audience members will be skeptical about your topic, when they disagree with you, or if they cannot make up their mind about where they stand on your main idea. The same with your credibility: they may be more skeptical about you and your credibility if they are uncertain about the topic or in disagreement with your position.

Persuading a skeptical audience is thus likely to require more "external" forms of evidence. What do we mean by "external"? These are sources outside the experience of the speaker. In the Rhetoric, Aristotle distinguished between "internal" and "external" sources of evidence. (Aristotle called internal proofs "artistic" proofs, and external proofs "inartistic.") The internal proofs come from the resources of the speaker; they include the credibility of the speaker, the speaker's feelings, and the speaker's logical inferences. External proofs -- outside the experience of the speaker -- included various forms of evidence. In contemporary rhetorical practice we might consider four basic forms of external proofs:

Expert testimony
Examples
Quanititative evidence
Audio/visual aids

The occasion for your speech may also motivate your choices of evidence. For instance, a lawyer presenting a legal case in a courtroom would probably need to meet different expectations than you would speaking in a classroom. Moveover, you may need to satisfy a different standard of evidence in your public speaking class than in your sociology class. In general, think of the 5R's as standards for testing evidence:

Relevancy: Is the evidence directly related to the topic at hand and pertinent to the rhetorical situation?

Recency: Is the evidence the most up-to-date information?

Reliability: Can the source of the evidence be trusted? Is the source unbiased? Did the person or group that developed the evidence use reliable research methods?

Recognition: Is the source of evidence familiar enough to the audience that it will grant credibility? Lack of recognition does not undermine the integrity of the evidence, per se. Yet, you will need to spend more time in your speech establishing the reliability and integrity of your evidence and its sources if your audience does not immediately recognize the source.

Rules of evidence pertinent to a discipline: Is the speaking situation governed by special rules of evidence? For instance, mathematical evidence used in a chemistry class might differ from the type of evidence used in a legislative chamber, or your English classroom.
Your speaking goals may also influence the type of evidence you will use. Consider the different general goals of speaking. An informative speech will likely require that you use evidence that amplifies your points; a persuasive speech will evidence as proof. Genres are different types of speeches. For instance, problem-solving speeches and eulogies are very different types.

Bibliography
Bibliography Roberts, W. Rhys (trans) The Rhetoric of Aristotle. At http://classics.mit.edu/Aristotle/rhetoric.html search on pull-down menu for Aristotle’s works.