Public Speaking

226 CHAPTER 16^ Foundations of Persuasion

buying a new car), “You can’t go wrong if you purchase a car like mine.” Your friends

then use their generalizations about cars and recommendations to make sense of what

you’ve just said.

Using enthymemes is inherently dialogical, for your listeners must form conclu-

sions based on their knowledge of what you don’t say. However, if they know nothing

about your subject (the rules regarding Bar Mitzvahs or the connection between debat-

ing and grades, for example), they’ll miss your meaning.

Testing Deductive Reasoning

There are two major tests for deductive reasoning:

1. For the conclusion to be valid, the premises must be true or highly probable.


2. To be reasonable, the conclusion must follow from the premise.

Reasoning Causally

Causal reasoning links two events or occurrences in such a way that one comes first and

regularly leads to the second. For example, the lack of oxygen to the brain (first event) is

followed by death (second event)—this link has been demonstrated time after time.

Cause–effect thinking is fundamentally engrained in the Euro-American belief

system where we actively seek reasons leading up to a particular phenomenon. Unlike

the lack of oxygen→death example, cause-and-effect links are less provable in complex

situations where many variables could be linked to an outcome. For example, what causes

women to be historically underrepresented in highly paid STEM (science, technology,

engineering, and math) careers? Are structures in place that don’t attract female students

to STEM courses? Do individuals, families, and society fail to provide encouragement

and support for young women to choose these careers? Do most women prefer learning

styles that mesh better with other majors? Many factors are involved. When reasoning

by cause, the key is to produce enough reasons to warrant the connection between the

two factors.

Here are just a few of the causal claims currently in the news: Chewing gum for

five minutes before a test can improve performance.^41 Ocean acidification inhibits the

development of shells and skeletal structures in a variety of sea animals. Sitting for hours

every day puts people at greater risk of heart disease and diabetes.^42

Testing Causal Reasoning

Test causation by asking a series of questions to assess whether the reasoning is valid.

1. Is there a real connection? Does one follow as a result of the first, or do the two
   events simply exist together in time? (Is it correlation or causation?)


2. Is this the only cause? The most important cause? Or are other factors at play?


3. Is the cause strong enough for the effect?
   In summary, you have at your disposal a variety of reasoning strategies, including
   figurative and literal analogies, inductive and deductive reasoning, and causal links.
   All of these reasoning types fall under the category of logos, or rational proofs.

Recognizing Logical Fallacies

A fallacy is a failure in logical reasoning that leads to unsound or misleading

arguments.^43 Fallacies have been around for thousands of years, as you can tell by

the Latin names given to some of them. As a speaker or critical listener, you should

examine arguments carefully to avoid using or being taken in by the common fallacies

shown in Table 16.1.

causal reasoning linking
two factors in such as way
that the first occurs before
the second and regularly
leads to the second as a
matter of rule

fallacy failure in logical
reasoning that leads to
unsound or misleading
arguments

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