“Bad reasoning as well as good reasoning is possible; and this fact is the foundation of the practical

side of logic.”

- Charles Sanders Peirce

Using Models of Reasoning

A Return to Logos

Reasoning from Specific Instances

Progressing from a number of particular facts to a general

conclusion.

This is also known as inductive reasoning.

FACT ONE: President Obama supported the American Recovery and Investment Act to allot $5 billion for early learning programs.

FACT TWO: President Obama launched the Educate to Innovate Campaign to enhance American students’ performance in key areas of science and math.

FACT THREE: President Obama endorsedthe Health Care and Education Reconciliation Act to invest more in higher education programs like community college.

CONCLUSION: President Obama values education.

Reasoning from Specific InstancesUse in Speech:

FACT ONE: I have done a lot of research on blood donation.FACT TWO: I have personally donated blood.FACT THREE: I sit with my grandma through her weekly

dialysis appointments.

CONCLUSION: I am credible to talk on the topic of blood donation.

Reasoning from Specific InstancesWhat to look out for:

Avoid hasty generalizations.Be careful with your wording (avoid absolutes).Provide additional evidence (statistics or testimony).

Types of inductive reasoningReasoning by Example: looks at one or more

cases within a specific class and reasons that if these cases have these features, than others will have those same features.

Example: “My friends Bill, Dianne, and Lynn were communication majors and they all got into law schools. Communication must be a good pre-law major.”

Causal ReasoningEstablishing a relationship between cause and effectExample:

CAUSE: The winter of 2010 yielded record snow falls in the Midwest.

EFFECT: The large volume of snow increased river levels which ultimately led many cities to flood.

Causal ReasoningWhat to look out for:

Avoid the post hoc fallacy.Don’t assume that events only have one cause.

Use in Speech:EFFECT: Obesity rates in America are alarmingly high.CAUSE: Studies have shown that a lack of exercise

contributes greatly to the problem of obesity.

Analogical ReasoningExample:

If you enjoyed watching the first seven Harry Potter movies, chances are you will enjoy the final one coming out this summer.

Analogical ReasoningComparing two similar cases and assuming what is true

of one should be true of the other.

What to look out for:Make sure that the two cases are, in fact, similar.

Analogical Reasoning Use in Speech:

A study done at the University of Iowa surveying 1500 college students found that 80% of students will take out a student loan some time throughout their time as an undergraduate. That means, that in this room, it is probable that at least sixteen of you are facing student loan date upon graduation.

Reasoning from PrincipleProgressing from a general principle to a specific

conclusion. This is also known as deductive reasoning.

Example:GENERAL PRINCIPLE: All fruits

grow on trees. MINOR PREMISE: Apples are fruits. CONCLUSION: Apples grow on

trees.

Reasoning from PrincipleUse in Speech:

GENERAL PRINCIPLE: The Children’s Miracle Network funds hundreds of hospitals that offer care specifically to children.

MINOR PREMISE: Sarah has been a patient at a Children’s Miracle Network hospital for over six months.

CONCLUSION: Your donation to the Children’s Miracle Network could benefit children just like Sarah.

Deductive SyllogismA syllogism contains three parts a major

premise, a minor premise, and a conclusion.

The major premise states a generalization, the minor premise relates a specific case and a conclusion is deduced from the two premises.

ExamplesMajor premise: All dogs have four legs.Minor premise: All animals have four

legs.Conclusion: All dogs are animals.

Major premise: All members of XYZ sorority have high grade-point averages.

Minor premise: Harriet is a member of the XYZ Sorority.

Conclusion: Harriet has a high grade-point average.

EnthymemesEnthymemes are “truncated” syllogisms with

a premise or conclusion unstated but implied.The omitted premise in the enthymeme is

supplied in the mind of the recipient.

Example: “Naturally he doesn’t know much about cars, he’s a professor.” (major premise is missing)

MoreExample: “All students are hardworking.”

(minor premise is missing)

Example: “Honor students aren’t interested in going to hockey games.Jim is a honor student. (Conclusion is missing)

CautionThe disjunctive syllogism: is characterized

by the major premise presenting two alternatives in an either/or relationship.

Example: “I will not study in the library this afternoon.Therefore, I will go to my room to take a nap.

“Either love America or leave it.”

MoreStructural validity vs. material truth.Careful observation of material truth of the

premises must be given in order to draw a logical conclusion.

Example: All cats have three legs.Felix is a cat.Felix has three legs.

Earning that Elusive *A*

The Successful Speech

Earning that A“A superior (A) speech stands out from the crowd. It has

superior content, excellent organization and distinctive delivery. In short, it represents the speaker’s best creative effort” (W p. 12).