Using Logic and evidence to Persuade 17.2 377
logicians, you reason inductively when you claim that an outcome is probably
true because of specific evidence.
For example, if you were giving a speech attempting to convince your au-
dience that foreign cars are not reliable, you might use inductive reasoning to
make your point. You could announce that you recently bought a foreign car
that gave you trouble. Your cousin also bought a foreign car that kept stalling on
the freeway. Finally, your English professor told you that her foreign car has bro-
ken down several times in the past few weeks. Based on these specific examples,
you ask your audience to agree with your general conclusion: Foreign cars are
unreliable.
As a persuasive speaker, your job is to construct a sound argument. That
means basing your generalization on evidence. When you listen to a persuasive
message, notice how the speaker tries to support his or her conclusion. The How
To box shows three questions you can use to judge the validity of a generalization
arrived at using inductive reasoning. Based on the answers to these three questions,
the logic in the example of problematic foreign cars is not particularly sound. You
would need considerably more evidence to prove your point.
reasonIng by analogy Reasoning by analogy is a special type of induc-
tive reasoning. An analogy is a comparison. This form of inductive reasoning
compares one thing, person, or process with another to predict how something
will perform and respond. In previous chapters we’ve suggested that using an
analogy is an effective way to clarify ideas and enhance message interest.
When you observe that two things have a number of characteristics in com-
mon and that a certain fact about one is likely to be true of the other, you have
Test the Validity of Inductive Reasoning
Ask these three questions:
• Are there enough specific instances to support the conclusion? Are three examples
of problems with foreign cars enough to prove your point that foreign cars are generally
unreliable? Of the several million foreign cars manufactured, three cars, especially if they are
of different makes, are not a large sample. If those examples were supported by additional
statistical evidence that more than 50 percent of foreign-car owners complained of serious
engine malfunctions, the evidence would be more convincing.
• Are the specific instances typical? Are the three examples you cite representative of all
foreign cars manufactured? How do you know? What are the data on the performance of
foreign cars? Also, are you, your cousin, and your professor typical of most car owners? The
three of you might be careless about routine maintenance of your autos.
• Are the instances recent? If the foreign cars that you are using as examples of poor reliabil-
ity are more than three years old, you cannot reasonably conclude that today’s foreign cars are
unreliable products. Age alone could explain the poor performance of your sample.How To