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Logic and Persuasion Handbook

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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

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:

Propositions

• 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


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


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 involves putting facts together to come up with a generalized statement as a conclusion.

Inductive Reasoning

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Logic

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.

Inductive Reasoning

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Logic

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.

Errors in Inductive Reasoning

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Logic

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.

Deductive Reasoning

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Logic

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.

Syllogisms

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Logic

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.

Errors in Deductive Reasoning

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Logic

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

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

For each argument below, identify whether inductive or deductive reasoning is used. Evaluate whether the conclusion is valid or invalid and explain your evaluation.

Exercise: Analyzing Logical Reasoning

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Logic


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


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

Write a short essay arguing a proposition. In your argument, use at least two examples each of valid inductive and deductive reasoning.

Exercise: Using Logical Reasoning

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Logic

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.

Logical Fallacies

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Logic

• 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.

Logical Fallacies

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Logic

• 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.

Logical Fallacies

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Logic

• 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.”

Logical Fallacies

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Logic

• 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.

Logical Fallacies

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Logic

• 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.

Logical Fallacies

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Logic

• 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.

Logical Fallacies

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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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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.

Propaganda

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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.

Propaganda

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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.

Ethical Reasoning

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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.

Ethical Reasoning

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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.

Identifying Unethical Persuasive Techniques

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Identifying Unethical Persuasive Techniques

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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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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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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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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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