Public Speaking

Use Logos or Rational Proofs (^225)

Reasoning Deductively

Inductive reasoning moves from specific examples to conclusions or generalizations,

but deductive reasoning goes the other direction. It begins with a generalization or

principle, called the premise, and moves logically to an application in a specific case.

(See Figure 16.1 for an example of the relationship between inductive and deductive

reasoning.) In formal logic, the deductive reasoning process is often shown in the form

of a syllogism such as this:

Major premise: Everyone who goes through a Bar Mitzvah is Jewish.

Minor premise: Aaron just had his Bar Mitzvah.

Conclusion: Therefore, Aaron is Jewish.

When you’re sure of the major premise, you can state your conclusion with

confidence. Because a Bar Mitzvah is the Jewish coming-of-age ceremony, only young

men in that religion go through it. In contrast, many premises are less certain. Although

some, such as “all people are mortal,” are 100 percent true, others, such as “This make

and model of car is good,” may not be valid in every case; a specific car might be a

“lemon.” So it’s wise to qualify both your premises and your conclusions. Here is an

example about the value of urban debate leagues:

Major premise: Participation in high school urban debate leagues helps many

students get better grades.

Minor premise: Yolanda Baylor is a debater at an inner-city high school in the

South Bronx.

Conclusion: She has probably improved her grades.^40

When you reason deductively, you rarely state the entire syllogism, so your

listeners must fill in the unstated premises. Aristotle called this an enthymeme. For

example, you might say, “Is Aaron Jewish? Of course. He just had his Bar Mitzvah,”

and let your audience make the necessary connections. Or (talking with friends about

deductive reasoning starting

with a principle (the premise)

and applying it to a specific

case

enthymeme omitting part of

the syllogism in an argument

and letting listeners supply

what’s missing; inherently

dialogical

Figure 16.1 

inductive and Deductive

reasoning You observe

a number of spaniels and

inductively reason that they

make good pets. Using that

premise, you deduce that

Curly, the specific spaniel

you’ve chosen, will be a

good family dog.

Spaniels Penny, Spot,

and Freckles are

good family pets.

Spaniels are

good family pets.

Spaniels are

good family pets.

This specific spaniel,

Curly, is probably going

to make a good family pet.

Conclusion

Conclusion

Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).
Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.