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Logic and
Persuasion
Handbook
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Logic and Persuasion
Handbook Menu
Persuasion
Logic
Ethical Reasoning and Propaganda
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Persuasion
Propositions
One of the main reasons people write and talk is
to persuade each other. Persuasive writing and
speaking attempts to convince someone of the
truth of a proposition, that is, a statement or
claim. There are four basic types of proposition:
•
A proposition of fact is a claim that certain
information is correct.
Candidate Wilkins comes from Illinois.
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Persuasion
Propositions
•
A proposition of value is a statement that a
feeling or judgment is valid.
Candidate Wilkins is a friendly woman.
•
A proposition about a problem combines fact
and judgment.
Candidate Wilkins is not qualified to run.
•
A proposition of policy is a claim that someone
should do something.
Everyone should vote for candidate Wilkins.
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Persuasion
Propositions
A proposition may be true or false. In evaluating
persuasive speaking and writing, you need to
know which type of proposition is being made so
that you can decide whether it is true or false.
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Persuasion
Evidence and Arguments
Persuasive writing and speaking usually includes
evidence, that is, reasons why someone should
accept a proposition. Together, a proposition and
a reason for accepting it make up an argument.
Everyone should vote for candidate Wilkins,
because she is the most qualified.
An argument may be valid or invalid, that is,
reasonable or unreasonable.
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Persuasion
Appeals
Arguments are meant to appeal to certain beliefs,
values, or feelings belonging to the reader or
listener. Most reasons given in support of a
proposition make at least one of four types of
appeal:
•
An appeal to logic is a claim based on fact
and reason.
Wilkins is unqualified, because she does not
meet the age requirement.
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Persuasion
Appeals
•
An appeal to ethics or values is a claim
based on shared values or judgments.
Wilkins is best, because she is the most
honest and caring.
•
An appeal to authority is a claim based on
sources believed to be reliable.
Wilkins is best, because the Metropolitan Bar
Association supports her.
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Persuasion
Appeals
•
An appeal to emotion is a claim based on
shared feelings.
Wilkins is best, because she has overcome
hardship.
In evaluating arguments, you need to know which
type of appeal is being made so that you can
decide whether it is valid or invalid. Note that an
argument may involve more than one type of
appeal.
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Persuasion
Exercise: Analyzing an Argument
Read the following statements. For each
statement, identify the type of proposition made
and the type of appeal used to support it.
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Persuasion
Exercise: Analyzing an Argument
1. If we want clean beaches, then we need to
provide trash cans and arrange for garbage
removal in the summer.
2. It is our responsibility as human beings to keep
ocean ecosystems healthy by polluting them
as little as possible or not at all.
3. According to eminent marine biologists, we
have a lot to learn about the animals that live
in the ocean depths.
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Persuasion
Exercise: Analyzing an Argument
4. Restricting owners of beachfront property
from building wherever they want to on their
property is highly unfair.
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Logic
Inductive Reasoning
Inductive reasoning involves putting facts together
to come up with a generalized statement as a
conclusion.
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Logic
Inductive Reasoning
Specific facts:
Fact 1. Star Wars is the second-biggest
money maker of all time.
Fact 2. The number one movie at the box
office in 2004 was Shrek 2.
Fact 3. Spider-man broke many box office
records in 2002.
Generalization: Science fiction and fantasy films
do very well at the box office.
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Logic
Errors in Inductive Reasoning
To avoid errors in inductive reasoning, be sure
you use a large enough sample of specific facts,
and of course, make sure your facts are accurate.
Assuming you have a large enough sample of
accurate facts, make sure that your generalization
is logical.
For example, it would be illogical to conclude from
the facts above that movies whose titles begin
with the letter S do well at the box office.
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Logic
Deductive Reasoning
Deductive reasoning is essentially the opposite of
inductive reasoning. With deductive reasoning you
start with a generalization to come to a conclusion
about a specific case.
Generalization: Paul can only eat vegetarian food.
Specific fact: The Glory Diner offers vegetarian
food.
Conclusion: Paul can eat at the Glory Diner.
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Logic
Syllogisms
A syllogism is a formal statement of a deductive
argument. It consists of a major premise, or
general statement; a minor premise, or related
fact; and a conclusion based on the two.
Major premise: People who travel between
countries need a passport.
Minor premise: Jody is flying from the United
States to Spain.
Conclusion: Jody needs a passport.
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Logic
Errors in Deductive Reasoning
Errors in deductive reasoning result from faulty
construction of the argument. Make sure the major
premise is a universal statement, that both premises
are true, and that the conclusion follows logically
from the premises.
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Logic
Errors in Deductive Reasoning
Note: A syllogism is valid if it follows the rules of
deductive reasoning. A syllogism is true if the
statements are factually accurate. Therefore, a
perfectly valid syllogism can be untrue. For example:
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Logic
Errors in Deductive Reasoning
Major premise: All voters are good citizens.
[There is more to good citizenship than voting.]
Minor premise: My parents are voters.
Conclusion: Therefore, my parents are good
citizens.
This conclusion is valid according to the premises;
however, it isn’t necessarily true because the major
premise is flawed.
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Logic
Exercise: Analyzing Logical Reasoning
For each argument below, identify whether inductive
or deductive reasoning is used. Evaluate whether
the conclusion is valid or invalid and explain your
evaluation.
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Logic
Exercise: Analyzing Logical Reasoning
1. An epic poem is a serious, long narrative poem
centered on the life of a cultural or national
hero or heroine. El Cid is an epic poem. In
more than 30,000 lines, it celebrates the life
and accomplishments of a Spanish military
and political leader who lived in the eleventh
century.
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Logic
Exercise: Analyzing Logical Reasoning
2. If a computer can play compact discs, the
computer must have been built before 1985.
This computer can play CDs. This computer
must have been built before 1985.
3. Many humorists use puns. Mark Twain used
puns in his writing and his speeches. Ogden
Nash used puns in his poems. Woody Allen
uses puns in his movies.
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Logic
Exercise: Using Logical Reasoning
Write a short essay arguing a proposition. In your
argument, use at least two examples each of valid
inductive and deductive reasoning.
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Logic
Logical Fallacies
A logical fallacy is a particular type of faulty
reasoning. Fallacies often seem reasonable at face
value, so they are often used, both intentionally and
unintentionally. Some fallacies are so common that
they have names.
To identify fallacies in the writing and speaking of
others and to avoid it in your own persuasive
communication, you need to be able to identify
fallacies and to understand why they are illogical.
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Logic
Logical Fallacies
•
Ad Hominem
Don’t listen to what Smyth says about the election;
he spent time in prison.
An ad hominem argument (literally, an argument
“against the person”) implies that a defect in a
person’s character or behavior is evidence that what
he says is unreliable. Note that the ad hominem
fallacy contains a hidden premise: People who have
spent time in prison cannot have valid opinions.
Because this premise is untrue, the argument about
Smyth is untrue also.
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Logic
Logical Fallacies
•
Non Sequitur or False Causality
This shirt is unlucky: every time I wear it,
something bad happens.
Non sequitur literally means “it doesn’t follow.”
Just because two events occur together, it
doesn’t follow logically that one caused the other.
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Logic
Logical Fallacies
•
Glittering Generalities
If you love freedom, vote for Jack.
Glittering generalities are words with
overwhelmingly positive connotations, used
to make it seem impossible to disagree with
an idea. How can you argue against the
idea of freedom? A listener’s initial reaction
to this statement might be, “Freedom is a
good thing, so I must vote for Jack.”
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Logic
Logical Fallacies
•
Overgeneralization and Stereotype
Tall people make excellent basketball players.
An overgeneralization is any conclusion that
may be accurate about a small group, but is
inaccurate when applied to a much larger
group. An overgeneralization about a group
of people is called a stereotype.
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Logic
Logical Fallacies
•
Argument from Authority and Celebrity
Endorsement
Four out of five doctors recommend Pumpidox
for most heart conditions.
Argument from authority is the quoting of an
alleged expert on a certain topic. As a logical
fallacy, arguments from authority rely solely on
the mention of the word “expert,” and give no
clear facts from the expert.
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Logic
Logical Fallacies
•
Argument from Authority and Celebrity
Endorsement
Four out of five doctors recommend Pumpidox
for most heart conditions.
Companies often hire celebrities to appear in
commercials for their products in the hope that
audiences will respond to the likeability of the
famous person, even if that person has no real
expert knowledge about the product.
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Logic
Logical Fallacies
•
The Bandwagon Effect
Choose America’s favorite toothpaste!
The term “jumping on the bandwagon” means
doing or thinking something because everyone
else is doing it or thinking it. This type of
reasoning provides no evidence to support a
decision or viewpoint.
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Logic
Logical Fallacies
•
Card Stacking
Senator Porter voted against childcare laws and
recycling programs. It’s time for new leadership!
Card stacking involves piling on evidence that
supports one side of an argument while ignoring
or suppressing valid evidence supporting the
other side. Saying that a politician voted against
positive-sounding programs does not mean that
he or she didn’t have good reason to, or that the
opposition has a better record.
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Ethical Reasoning and Propaganda
Propaganda
Propaganda is the process of persuading by
deliberately misleading or confusing an audience.
Through the use of combinations of logical
fallacies, propaganda can appeal to ethics or
values, authority, or emotion, but they do so in a
way that is unsupported or inappropriate.
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Ethical Reasoning and Propaganda
Propaganda
Political propaganda
A vote for Marmelard is a vote for the enemy!
America: You’re with us or against us!
Advertising
Be the best parent you can be: Serve your kids
Super Goody cereal.
The most successful people shop at Blorland’s
Department Store.
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Ethical Reasoning and Propaganda
Ethical Reasoning
Reasoning that persuades by helping its intended
audience make informed decisions is called
ethical reasoning. As a writer or speaker, you
have the responsibility to use ethical reasoning
and avoid propaganda. This means that you must
gather complete information about a topic, check
your facts for accuracy, and make sure that your
reasoning includes no errors in logic or false
conclusions.
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Ethical Reasoning and Propaganda
Ethical Reasoning
You should address opposing evidence with
clear and accurate argumentation. Using ethical
reasoning in your persuasive writing or speeches
will strengthen your positions as your audience
sees that you have logically addressed all sides
of an idea.
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Ethical Reasoning and Propaganda
Identifying Unethical Persuasive Techniques
The following essay contains several examples
of faulty reasoning. Read through the entire text
once, then go back and look for logical fallacies,
invalid arguments, and manipulative appeals. For
each example you find, make an entry in a chart
like the one shown on the next slide. Then write
a paragraph evaluating the essay’s argument.
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Ethical Reasoning and Propaganda
Identifying Unethical Persuasive Techniques
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Ethical Reasoning and Propaganda
Don’t Take Away Our Freedom
The school board recently announced plans
to remove all vending machines from our
schools’ cafeterias. They say that candy, snacks,
and cola are bad for students. But is starvation
good for students? Is taking away freedom to
choose good for students?
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Ethical Reasoning and Propaganda
Don’t Take Away Our Freedom
Every expert on nutrition agrees that it is not
healthy for kids to go for hours between meals
without some sort of snack in between to tide
them over. If the school board has its way,
students will be passing out at their desks from
hunger and dehydration.
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Ethical Reasoning and Propaganda
Don’t Take Away Our Freedom
Principal Spaly claims that students are more
likely to pass out from a “sugar crash.” This is
the same Principal Spaly who recently showed
what he thought of students when he denied
sophomores the right to park at the high school.
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Ethical Reasoning and Propaganda
Don’t Take Away Our Freedom
We are taught in these very schools that
America is a land of democracy, freedom, and
liberty. It is clear that the school board has
forgotten this. Any student who loves his or her
school will write to the school board and let them
know how we feel.
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