Defining Evidence

Evidence refers to the available body of facts or information indicating whether a belief or proposition is true. The facts and information affirm the validity of the idea. To understand it in the opposite, to lack evidence is to lack the validity of a belief or idea. Evidence builds credibility.

As the name suggests, evidential appeals rely on the use of evidence to persuade the audience. Evidential appeals can be used in both emotional and logical appeals, though the method for delivery differs. The success or failure of the appeal depends on how well the evidence has been defined and laid out for the audience.

In the context of persuasive speaking, evidence can be evaluated for its persuasive ability in the following three ways:

1. Accuracy: The evidence must be truthfully constructed and defined. For example, if an argument hinges on the premise that grass is purple, no rhetorical technique will be able to persuade the audience. Evidence must be accurate to be credible, as its credibility rests on its accuracy.
2. Audience Understanding:Evidence must be presented completely, but in a manner that the audience can comprehend. For example, an evidential appeal that uses rising carbon dioxide levels as evidence for stricter pollution regulation will not be effective if the audience does not know what carbon dioxide is or why it is bad for the environment. The audience must be able to understand the evidence before it is used in an appeal.
3. Relevant Context: The evidence must be defined within the context of the appeal. A textbook definition of the different types of bonds between the atoms of carbon dioxide is not relevant information for why the tax rate should be lowered.

Information used in evidential appeals must serve two purposes at once. First, it must be complete enough that it strengthens the appeal. If the evidence is weak, incomplete, or irrelevant, it does not help the appeal, and may even hurt its persuasiveness. Secondly, the evidence must be defined only as comprehensively as necessary. The purpose of an evidential appeal is to persuade the audience; overwhelming the audience with too much information or evidence may only confuse them.