Avoiding Errors in Logic

Fallacies of Logic

To avoid these problems, try to be objective about your subject matter in general, and learn to recognize common errors in reasoning so you can spot your own.

The following is a discussion of several classical fallacies of logic. You can use your knowledge of these errors to refute the arguments of your opposition as well.

Types of Fallacies

• Fallacies of Deduction (or Inference)
• Fallacies of Induction
• Fallacies of Irrelevance
• Fallacies of Imprecision
• Fallacies of Misrepresentation
• Some Other Fallacies and Pitfalls of Logic

Fallacies of Deduction (or Inference)

1. Non sequitur -- "It does not follow" -- an argument in which the conclusion is not a necessary consequence of the premises.

   Faulty Billy Joe is honest; therefore, he will land a good job.

   Better Billy Joe is honest; this quality should help him land a good job.

2. Self-Contradiction -- an argument that contains mutually exclusive premises.

   Faulty The government should control this unmanageable situation

   Better We cannot expect government to control the uncontrollable, but we can expect them to resolve this situation.

3. Circular Reasoning, or Begging the Question -- a deductive argument in which the conclusion is contained in one of the premises.

   Faulty I believe that this X is evil because society has always condemned it; society has always condemned it because it is evil.

   Better Society has always condemned this X as an evil, and I believe society's judgment is correct.

Fallacies of Induction

1. Confusion of Fact and Value Judgment -- What can be observed, measured, and tested is a fact. Whether we like the fact or not, whether we believe is should be changed or not, these are valued judgments, meaning they are opinions or personal preferences. Both value judgments and facts are important, but they should not be confused.

   Fact and Value Confused
   Your hair is too long. ("Too" indicates a personal preference, not a fact.)
   Fact Your hair is long.
   Value I don't like your hair long.
   
   Fact and Value Confused
   People who watch Entertainment Tonight are unintelligent.
   Fact Many people, intelligent as well as unintelligent, watch ET.
   Value ET is of no real news value, and therefore cannot be regarded as worthwhile viewing.

2. Hasty Generalization -- a generalization offered on the basis of too little evidence or evidence that is exceptional or biased in some way. Stereotyping is a type of hasty generalization. Enough evidence must be gathered to warrant generalizing, and the evidence must not be exceptional or unusual.
   • Insufficient Evidence -- None of the children in my family drink coffee; therefore, children don't like coffee. (More evidence is needed before this generalization is warranted.)
   • Exceptional or Unusual Evidence -- The increasing number of subway riders in Boston, New York and Washington, DC shows that urban dwellers in this country prefer mass transit to the automobile. (These three cities are unique in that their geographic area favors mass transit. Cities such as Los Angeles or Oklahoma City, whose geographic area is very large and whose population density is sparse, might not favor mass transit.)

3. Post hoc, ergo propter hoc = "After this, therefore because of this" -- the mistake of assuming that because one event followed another, the first must be the cause of the second.

   Faulty Liz got wet and cold in the rain, so now she has a cold. (Many people become cold and wet in the rain and do not catch a cold. )

   Better Liz got wet and cold in the rain, causing her resistance to be weakened and making her susceptible to the rhino virus she came in contact with, and now she has a cold.

4. False Analogy -- a weak, even far-fetched comparison; using a comparison in which the differences outweigh the similarities, or in which the similarities are irrelevant to the claim the analogy is intended to support

   Faulty The new mayor is not even the head of his own household, so I do not expect him to be a good civic leader or to have much influence on the city council.

   Better Because he was indecisive during his campaign, I do not expect the new mayor to be a good civic leader or to have much influence on the city council.

Fallacies of Irrelevance

1. Ignoring the Question, or Rambling -- presenting details or facts that are off the point and do not support the thesis.

   Faulty We should do more to help the poor help themselves. Of course, the Bible says we'll always have the poor with us, even though it does not say we should give them everything we have. (Here the writer loses sight of the main point: We should help the poor help themselves.)

2. Appeal to Ignorance -- assuming that an argument is valid simply because one has found no evidence to the contrary; insisting that a claim or theory must be right because no one has proven it wrong. This fallacy relieves the person who makes the claim from having to support or prove the point by simply shifting the burden of disproof to the opponents.

   Faulty Wearing a copper bracelet will help relieve the pain of arthritis. Doctors may say the bracelet does not help, but they haven't proven that it doesn't. (translation = "If you can't prove me wrong, then I must be right.")

3. Ad hominem = "To the person" -- an attempt to disprove an argument by attacking the person who presents it. Do not evade the facts by attacking your opponent's economic, social, philosophical, or ethnic background.

   Faulty That merchant is allegedly a thief and a liar; her argument against a sales tax are worthless. (The merchant might steal and lie and yet have excellent views on economic matters such as a sales tax. The evidence is not relevant to the assertion.)

4. Ad populum = "To the people" -- an appeal to popular emotions, prejudices, or beliefs.

   Faulty The majority of Americans today are a generous, compassionate, and freedom-loving people; to reflect the will of the people, immigration laws should not be changed but abolished.

   Faulty Immigrants are robbing good, honest, hard-working Americans of jobs they need to support their families. For this reason immigration laws should be changed so that we can limit the number of people who come into our country.

5. Bandwagon, or "Join the Crowd" -- an argument saying, in effect, "Everyone's doing or saying or thinking this, so you should too."

   Faulty This novel has been number one on the best-seller list for weeks. You have to read it!

6. Appeal to Authority, or Appeal to Prestige: -- an argument relying not on facts but on opinions, beliefs, or theories of experts or on testimonials of famous people.

   Faulty Both the Washington Post and the New York Times have predicted Jones' reelection, so he will represent us again. (Prediction, even by experts, may or may not be accurate.)

   Faulty Scientists say that our planet is already experiencing the effects of global warming.
   This fallacy is often called the Appeal to the False or Irrelevant Authority -- an argument citing the opinion or preference of person who has no expertise about the subject. Think about the TV commercials that attempt to persuade you to buy a product simply because it is endorsed by a famous person.

   Faulty Grant Hill eats this cereal, so it is probable more nutritious than the others.

   Better A comparison of nutrition information printed on the boxes indicates that this cereal is probably more nutritious than the others.

Fallacies of Imprecision

1. Ambiguity -- a statement or argument in which the meaning is unclear; two or more different interpretations are therefore possible.

   Ambiguous John is a poor mechanic. (This statement could be interpreted as "John is not a competent mechanic" or as "John's financial resources are limited.")

2. Equivocation -- a statement or argument in which an expression or word is used in two different senses.

   Faulty We Americans have the right to pursue happiness, and we should want to do what is right. So let's make happiness our goal in life. (The word "right" is used in two different senses.)

Fallacies of Misrepresentation

1. Oversimplification -- a statement or argument that leaves out relevant considerations about an issue.

   Faulty People who pass tests are usually lucky.

   Better People who pass tests are usually lucky, although they often are prepared to answer most questions well.

   Faulty World War I was caused by the assassination of Archduke Francis Ferdinand in June 1914.

   Better World War I had many causes, but the immediate precipitating event was the assassination of Archduke Francis Ferdinand in June 1914.

   Faulty Students are graduating from high school illiterate because of poor teaching.

   Better Students are graduating from high school illiterate for a number of reasons including a lack of student preparation, a lack of parental supervision, or a lack of teacher competence.

2. Sweeping Generalization -- Closely related to the fallacy of oversimplification is the sweeping generalization, a statement or argument that fails to acknowledge important qualifiers or limiting factors.

   Faulty Everyone supports Political Correctness. (This writer does fails to acknowledge that not "everyone" does support "all" aspects of the PC movement.)

   Faulty Any intelligent, sensitive person supports Political Correctness. (This statement not only generalizes about those who support PC, but it also implies that if the person does not support PC, then he/she is not intelligent or sensitive.)

3. False Dilemma (aka False Division, aka Either-Or Fallacy) -- any attempt to eliminate the middle ground by drawing a sharp distinction between parts of a complex whole when the facts show a gradation between the parts.

   Faulty All living things are either plants or animals.

   Faulty A nation is either at war or at peace.

   Faulty We either accept the demands of terrorists or ignore them completely.

4. Card-Stacking -- ignoring or deliberately withholding evidence on the other side of the question, evidence that may weaken the writer's position or that may be hard to refute.

Some Other Fallacies and Pitfalls of Logic

1. Red Herring -- To divert attention from the real issue, a person may sometimes introduce a false, often emotional, yet irrelevant, issue to lure discussion away from the real point. For instance, a store owner, trying to soothe a customer whose order had been delayed, attempted to excuse his own neglect by saying, "You know, you can't depend on the long-haired, ear-ringed, freaky help you get these days." He attempted to evade responsibility for the delay by using a red herring, the customer's own possibly negative attitude toward young people.

2. Attacking a Straw Man -- There are times when someone distorts or exaggerates the content of someone's statements to try to destroy his/her logic through misrepresentation. Just as it is difficult to knock over a real man, it is difficulty to attack a sound, logical position in an argument. But since it is easy to knock over a man made of straw, it is easy to attack someone's position after it is distorted to suit one's purpose. Like the argument ad hominem , which diverts attention form a strong argument by dragging in personal traits, attacking a straw man distorts a strong argument by replacing it with a weaker one which can be assailed more easily. The weaker one can be an exaggerated version, a distortion, or sometimes an outright lie. Such a device is common in discussions of controversial issues such as abortion, capital punishment, or rape. If someone says she does not believe that capital punishment is effective in deterring murder, you would be attacking a straw man if you reduced the seriousness of her statement with "I bet you think we should send killers back on the streets with just a slap on the wrist." Belittling someone's idea is, unfortunately, quite a common way to try to destroy someone's credibility.

3. Name-Calling -- the presentation of an issue in emotionally "loaded" language often with insulting racial, religious, or political slurs which interfere with an objective, logical view of the subject. An example arises from the label "Irish Mafia," which was sometimes pasted on the Kennedy clan. And here is a passage from a "White Power" newsletter written in very loaded language:
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     Rodney Bingenheimer is running the latest youth sensation in Los Angeles: Rodney's Discotheque on Sunset Strip. . . . Many who are dressed as girls are really boys and vice-perverse-a.

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