Using Logic and evidence to Persuade 17.2 383
• Deductive. If you reason deductively (from a generalization to a specific
conclusion), you need evidence to document the truth of your initial
generalization.
• Causal. When developing an argument using causal reasoning, evidence is
vital to establish your message that one or more events caused something to
happen.
Evidence in persuasive speeches consists of facts, examples, statistics,
and expert opinions. (Recall that in Chapter 8 we discussed using these types
of supporting materials in speeches.) Aristotle called logic logos, which means
“the word.” Using words as well as statistical information to develop logical
arguments can make your persuasive efforts more convincing. It can also clarify
your own thinking and help make your points clear to your listeners.
FaCts When using facts to persuade, make sure your fact is really a fact. A
fact is something that has been directly observed to be true or can be proved
to be true. The shape of the earth, the number of women university presidents,
and the winner of the 2014 Super Bowl have all been directly observed or
counted. Without direct observation or measurement, we can only make an
inference. An inference is a conclusion based on available evidence or par-
tial information. It’s a fact that sales of foreign-made cars are increasing in the
United States; it’s an inference that foreign-made cars are always the highest-
quality cars.
examples examples are illustrations that are used to dramatize or clarify a
fact. For example, one speaker, in an effort to document the increased violence
in children’s television programs, told her audience, “Last Saturday morning, as
I watched cartoons with my daughter, I was shocked by the countless times we
saw examples of beatings and even the death of the cartoon characters in one
half-hour program.” The conclusion she wanted her audience to reach: Put an
end to senseless violence in children’s television programs.
Only valid, true examples can be used to help prove a point. A hypotheti-
cal example, one that is fabricated to illustrate or clarify a point, should not
be used to reach a conclusion. It should be used only to clarify. David encour-
aged his listeners to join him in an effort to clean up the San Marcos River. He
wanted to motivate his audience to help by asking them to “imagine bringing
their children to the river ten years from now. You see the river bottom littered
with cans and bottles.” His example, while effective in helping the audience
to visualize what might happen in the future, does not prove that the river
ecosystem will deteriorate. It only illustrates what might happen if action isn’t
taken.
opInIons Opinions can serve as evidence if they are expressed by an expert,
someone who can add credibility to your conclusion. The best opinions to
use in support of a persuasive argument are those expressed by someone