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# Thread: The 15 Fallacies of Logical Reasoning

1. ## The 15 Fallacies of Logical Reasoning

The following classic errors of reasoning appear with some frequency.

1. Uncertain Use of a Term or Concept: the PUN

“Some people claim that the values that this country was built on are now being ignored by modern-day corporations. But this is incorrect. Corporations are purely profit-driven enterprises, beholden only to their shareholders, and as such they can only assess objects based on their value.”

“Tomorrow you shall find me a grave man.”

1. Argumentum Ad Hominem a.k.a Source Argument:

“The anti-smoking views expressed by Senator Smith should be ignored. After all, Smith himself is a smoker!”

“How can we appoint her as the next asst. manager? After all, she’s a divorcee. If she couldn’t even manage her marriage, how would she manage this company?”

A source argument can take different forms, including the following:
1. Focusing on the motives of the source.
2. Focusing on the actions of the source (as in the above example).

1. Circular Reasoning:

In circular reasoning the author assumes as true what is supposed to be proved.
Consider the following example:
“This essay is the best because it is better than all the others.”

“Ram himself said that he is an honest man, and an honest man would never lie. Therefore, Ram is honest.”

“All men are rational. Socrates is a man. Therefore, Socrates is rational.”

“All cats are dogs. Therefore, some cats are dogs.”

In this example the premise and the conclusion are identical in meaning. As we know, the conclusion should always follow from the premise. In the example above, the premise supports the conclusion, but the conclusion equally supports the premise, creating a “circular” situation where you can move from premise to conclusion, and then back again to the premise, and so on. Here is another example: “I must be telling the truth because I’m not lying.”

1. Errors of Conditional Reasoning:

Note that the authors can either mistake a necessary(required) condition for a sufficient(assured) condition, or mistake a sufficient condition for a necessary condition:

A à B is true
Mistaken Reversal: Bà A is true.
Mistaken Negation: ~A à ~B is true.

1. Mistaken Cause and Effect:

1. Post Hoc Ergo Propter Hoc: Assuming a causal relationship on the basis of the sequence of events. The author “mistakes the observation that one thing happens after another for proof that the second thing is the result of the first”; or, it “mistakes a temporal relationship for a causal relationship”

“Ever since a new principal has been appointed, seven members of faculty have quit without any explanation whatsoever. I am convinced that they quit because of the new principal.”

“I wrote my mid term papers last month, but with poor preparation. I had expected to fail. However, I was shocked to find out that I topped my class. Just as I was about to give up on the reason for the strange occurrence, I remembered that my friend Ram had given me a rabbit’s foot before entering the examination hall. He had told me that it would bring me good luck. Now I’m convinced that it really works.”

“In the Great Depression of the 1930’s, in the United States, some people had observed that periods of business expansion were preceded or accompanied by rising prices. From this, they concluded that the appropriate remedy for the depression was to raise wages and prices. This idea led to a host of legislation and regulations to prop up wages and prices in an inefficient manner. Did these measures promote economic recovery? Almost surely not. Indeed, they probably slowed recovery, which did not occur until total spending began to rise as the government increased military spending in preparation for World War II.”

2. It can be defined as the error of assuming a causal relationship when only a correlation exists; “confusing the coincidence of two events with a causal relation between the two”; or “assumes a causal relationship where only a correlation has been indicated.”

“It has been observed that whenever we launch a new product into the market, there’s a slump in business. Therefore, we must wait a while to increase our profits before lauching our next innovation.”

3. Failure to consider an alternate cause for the effect, or an alternate cause for both the cause and the effect. “Fails to exclude an alternative explanation for the observed effect” “overlooks the possibility that the same thing may causally contribute both to education and to good health”

4. Failure to consider that the events may be reversed. “The author mistakes an effect for a cause”

1. Straw Man:

This error occurs when an author attempts to attack an opponent’s position by ignoring the actual statements made by the opposing speaker and insteaddistorts and refashions the argument, making it weaker in the process. Infigurative terms, a “straw” argument is built up which is then easier for the author to knock down.

Often this error is accompanied by the phrase “what you’re saying is” or “if I understand you correctly,” which are used to preface the refashioned and weakened argument. Here is an example:

Politician A: “The platform proposed by my party calls for a moderate increase in taxes on those individuals making over \$20,000 per year, and then taking that money and using it to rebuild the educational system.”
Politician B: “But what you’re saying is that everyone should pay higher taxes, and so your proposal is unfair.”

Ram: “I believe capital punishment should be abolished everywhere as it is inhumane. Why take away something from someone that we cannot restore? Only God has the right to give or take life, and we’re not God.”
Shyam: “What? How can you say such a thing? Abolish punishment? So you mean that criminals should run free?”

An internal contradiction (also known as a self-contradiction) occurs when an author makes conflicting statements. The example used was:

“Everyone should join our country club. After all, it’s an exclusive group that links many of the influential members of the community.”

The self-contradiction occurs when the speaker says “Everyone should join” and then follows that by saying that it is “an exclusive group.” Exclusive, by definition, means that some people are excluded.

1. Appeal Fallacies:

While there are a number of “appeal” fallacies that appear in traditional logic (Appeal to Emotions, People, Authority, Force, and Tradition, etc.), the following three are the most applicable.

1. Argumentum Ad Vericundium a.k.a. Appeal to Authority.
An Appeal to Authority uses the opinion of an authority in an attempt to persuade the reader. The flaw in this form of reasoning is that the authority may not have relevant knowledge or all the information regarding a situation, or there may a difference of opinion among experts as to what is true in the case. Here is an example:

“World-renowned neurologist Dr. Samuel Langhorne says that EZBrite Tooth Strips are the best for whitening your teeth. So, you know if you buy EZBrite you will soon have the whitest teeth possible!”

The primary defect in this argument is its use of a neurologist as an authority figure in an area of dentistry. While Dr. Langhorne can reasonably be appealed to in matters of the brain, dental care would be considered outside the scope of his expertise.

2. Argumentum Ad Populum a.k.a. Appeal to Popular Opinion/Appeal to Numbers

This error states that a position is true because the majority believes it to be true. As you know, arguments are created by providing premises that support a conclusion. An appeal to popular opinion does not present a logical reason for accepting a position, just an appeal based on numbers.

3. Appeal to Emotion
An Appeal to Emotion occurs when emotions or emotionally-charged language is used in an attempt to persuade the reader. Here is an example:

“Officer, please do not give me a ticket for speeding. In the last month I’ve been fired from my job, kicked out of my apartment, and my car broke down. I don’t deserve this!

1. Survey Errors:

Surveys, when conducted properly, produce reliable results. However, surveys can be invalidated when either of the following three scenarios arise:

1. The survey uses a biased sample.
2. The survey questions are improperly constructed. If a survey question is confusing or misleading, the results of the poll can be inaccurate.
3. Respondents to the survey give inaccurate responses.

1. Errors of Composition and Division:

Composition and division errors involve judgments made about groups and parts of a group. An error of composition occurs when the author attributes a characteristic of part of the group to the group as a whole or to each member of the group. Here is an example:

“Every party I attend is fun and exciting. Therefore, my life is fun and exciting.”

The other form of this error occurs when the author attributes a characteristic of the whole (or each member of the whole) to a part of the group. Here is an example:

“The United States is the wealthiest country in the world. Thus, every American is wealthy.”

1. False Analogy:

A False Analogy occurs when the author uses an analogy that is too dissimilar to the original situation to be applicable. Here is an example:

“Just as a heavy rainfall can be cleansing, the best approach to maintain a healthy relationship is to store up all your petty grievances and then unload them all at one time on your partner.”

The comparison in the example fails to consider that a heavy rainfall and an emotionally charged situation are fundamentally different.

“One must understand that the massacre of the Sioux by the early American settlers was totally justified even if it was cruel. After all, you can’t make an egg without breaking a few eggs.”

1. False Dilemma:

A False Dilemma assumes that only two courses of action are available when there may be others. Here is an example:

“Recent accidents within the oil industry have made safety of operation a critical public safety issue. Because the industry cannot be expected to police itself, the government must step in and take action.”

The argument above falsely assumes that only two courses of action exist: industry self-policing or government action. But this ignores other courses of action, such as consumer watchdog groups.

1. Red Herring a.k.a Shiftin the burden of proof:

In this falacy, the author evades the bone of contention and reasons his stand by talking about things irrelevant to the discussion or argument. Here is an example:

Ram: “In order to dissuade teenagers from smoking, the government should impose a fine on any retailer who sells cigarettes to minors.”
Shyam: “The government had placed on a similar fine on the sale of alcohol to minors, but that didn’t work. I don’t see why your suggestion should.”

Traffic Policeman: “The fine for riding on the wrong side of the road is Rs. 200. Please pay up!”
Ram: “Sir, how can u charge me such a huge amount for something so insignificant, when there are people doing all sorts of nonsense on roads, like riding with 3 people on a bike, or ignoring traffic lights, or even driving without a license. I’ve not done anything anything that serious!”

1. Hasty Generalisation or Unwarranted Assumption:

When there’s no correlation between the premise and conclusion in an argument, it’s called a hasty generalisation. Here is an example:

“I’ve met 5 Australians in my life, and they were all tall. Hence, Australians are a very tall people.”

“The Democrats won the previous elections, therefore, they will the next one too.”

1. Slipery Slope:

When an author makes multiple unwarranted assumptions, he ends up miles away from his original premise. Here’s an example:

“The human population has been ever increasing. No one’s really worried about the consequences. Every year thousands of acres of forest lands are lost to infantile civilisations. At this rate we will have no place to cultivate crops. All the natural resources will be depleted. Man will start eating man, just to stay alive, and we will go back to being savages as before!”

2. ## The Following 3 Users LIKE this post from saloni:

ayush05 (21st February 2013), sciengprof (1st June 2011), skan1292 (27th September 2012)

3. ## Re: The 15 Fallacies of Logical Reasoning

wow...I never knew it that there are several ways of reasoning.

4. ## The Following User LIKES this post from sciengprof:

skan1292 (27th September 2012)

5. ## Re: The 15 Fallacies of Logical Reasoning

Tough to understand this....

6. ## Re: The 15 Fallacies of Logical Reasoning

can you plz suggest some good study material for cat..?

7. ## Re: The 15 Fallacies of Logical Reasoning

arun sharma is a great book to follow

8. ## Re: The 15 Fallacies of Logical Reasoning

can u pls suggest me the books for the cat preparition

hi.saloni ..