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Avoiding Faulty Logic

May 23, 2011

Good writing requires good reasoning and the ability to avoid logical mistakes, or logical fallicies. The ability to think critically also protects from manipulation by those skilled in the art of rhetoric.


Students: As a group, let’s make a list of logical mistakes. Research a common logical mistake and write about it. Give an example, so that other students can also understand more clearly. There are several good websites, but you may like this one, or this one. Also, note any cultural differences that may exist.

Here is my example: Ad Hominen (Personal Attack)

This logical mistake occurs when someone attacks the speaker personally and not the speaker’s argument.

Fat guy: “The burgers I make at home are very healthy.”

Fit guy: “What? How can you say that your burgers are healthy? You’re so fat.”


Explanation: The fit guy attacks the fat guy personally by negatively commenting on the fat guy’s weight, not on his opinion that he (the fat guy) makes healthy hamburgers at home. BTW, burgers can be healthy depending on the definition of ‘healthy’, ingredients, cooking methods, etc. Also, we do not know why the fat guy is fat. He may be fat for reasons other than food choices.

44 Comments leave one →
  1. proftodd permalink*
    May 23, 2011 6:04 pm

    Here are some more examples of logical mistakes:

    An appeal to pity occurs when the speaker tries to persuade the listener by using pity.

    Student: “Teacher, I’m sorry that I missed the deadline. I’ve been sick.”

    Appeal to popular
    Many people listen to hip hop. Therefore, hip hop must be good music.

    Explanation: Just because something is popular does not mean it is good. Think about how popular McDonald’s food is. It may be popular for other reasons, including price, marketing, accessability or location, and so on.

    • May 23, 2011 11:25 pm

      I am familiar with this ‘appeal to pity’ —

      • proftodd permalink*
        May 25, 2011 5:29 pm

        As am I. Are you, by any chance, a teacher?

  2. Olivia permalink
    May 24, 2011 9:49 pm

    Argumentum ad Ignorantiam

    People often insist some fact true because it is not proved to be not true.

    For example –

    A: Does ghost really exist?
    B: Can you demonstrate the ghost is not exist?
    A: No,,,
    B: Then, the ghost is really exist.

    • proftodd permalink*
      May 25, 2011 5:09 pm

      Yes, or better known as the appeal to ignorance. You can explain it more simply as shifting the burden of proof to the wrong side. For example, if someone says that ghosts exist, they should have the burden of proving their position by providing proof, i.e., the person who says that ghosts exist should have to provide evidence of the existence of ghosts. They should not shift the burden of proof to the other side; the other side should not have to provide evidence that ghosts do not exist.

  3. Yang woochan - critical reading and writing2 permalink
    May 24, 2011 11:02 pm

    Appeal to Novelty
    An appeal to novelty is the opposite of an appeal to antiquity. Appeals to novelty assume that the newness of an idea is evidence of its truth. They are thus also related to the bandwagon fallacy.

    Here is my example : Appeal to Novelty

    (1) String theory is the most recent development in physics.
    (2) String theory is true.

    (1) Religion is old-fashioned; atheism is a much more recent development.
    (2) Atheism is true.

    Explanation: That an idea is new certainly doesn’t entail that it is true. Some ideas that are new now will surely go the same way. Each of these arguments commits the appeal to novelty fallacy. The former takes the newness of string theory to be evidence that string theory is true; the latter takes the newness of atheism to be evidence that atheism is true. Merely being a new idea, of course, is no guarantee of truth. The newness of string theory and atheism alone, then, should not be taken to be evidence of the truth of these two positions.

    • May 24, 2011 11:41 pm

      Of course you could counter the religion argument by stating that because the religion has been accepted for thousands of years by millions of people, it must be true.

      Science however maintains stronger controls on deciding whether something is legitimate or not. It requires plenty of tests before it is even considered as an accepted theory. Still, plenty of scientific theories have been disproven even after these tests, especially in the field of medicine.

  4. proftodd permalink*
    May 25, 2011 5:24 pm

    Hasty Generalizations
    Student A gets all A+’s in her classes. Student B gets all C’s in his classes.

    Conclusion: Student A is intelligent. Student B is not intelligent.

    Explanation: There are various reasons for a student’s grade, including but not limited to effort, ability, aptitude, and motivation. Student B may be intelligent, but he may have issues that interfere with proper study, preparation, and test taking. We do not have enough information to draw a conclusion about student B’s intelligence. Student B could have health issues, family problems, motivational problems, poor study habits, bad strategies, ineffective test taking skills, a lack of time to commit to studying, and so on.

  5. MIN.Y.K permalink
    May 25, 2011 6:28 pm


    People have a binary system, yes or no. People consider all things dividing into two ways. I know this system already confirmed in Korea. In the past, it is very severe in dictatorical government. In 1980s, We believed red color meaning socialism. It is like a tabbo to say about government. I think it is old stereotype.

    Example : When I was young, some adults had a question “Which one do you like mother or father?” It was the most difficult question in my life. I couldn’t dicide. If I answered this problem, I might be embarrased in many times. Mother or father used this answer to make a value determined criterion. But, many people like this question. Please, do not ask.

    • proftodd permalink*
      May 26, 2011 10:10 am

      Yes, these are Either/or Arguments. See page 123 of our textbook.

  6. May 25, 2011 11:27 pm

    In the hot issue tv program “I’m singer”,
    Singers were ranked by audience through the contest of their song or famous song.
    That contest means lose or win , survive or fail.
    Especially on media, ranking is a standard to evaluate the singers abillity.

    explanation : But it is a logical fallacy. Because ranking just lead audience for getting a popularity of their program, and because of fans of differnt maniac, age,
    singer’s different abillity in their part must not be taken easily by rank.
    That is, it is not a logical standard to list.

  7. anonymous permalink
    May 26, 2011 2:07 pm

    Faulty Causation

    When a research shows that both ice-cream and air conditioner sales rates increases at the same time in July, some people may conclude that ice-cream sales rate has influenced the sales of air conditioner or vice versa.

    Explanation: The two sales rates may be closely related to each other, because the season is summer in July. People usually buy more ice creams and air conditioners in summer than in other seasons. However, the ice cream sales and the air conditioner sales is not cause-effect relation. In fact, what does affect the two sales rates is the weather.

    • proftodd permalink*
      May 26, 2011 9:01 pm

      Specious Reasoning

      I love this funny clip from the Simpson’s TV show. The background to the clip is as follows; Homer (the idiot father) wanted to keep bears away from the town, so he arranged a bear patrol, which means people who look out for bears. Homer doesn’t see any bears, so he thinks that the bear patrol is working, i.e., there are no bears because of the patrol. His young daughter Lisa is very intelligent and she illustrates his poor logic. She says he is using specious reasoning. He says, “Thank you.” Haha. Unfortunately, he doesn’t his logical error.

      • anonymous permalink
        June 7, 2011 2:33 pm

        oh, it took some time for me to understand the animation. Now I understand why you posted the clip. It’s an extreme example.
        Does this work?
        ” I’m smartest in the world, because I’ve never met anyone smarter than me.”

  8. Chu permalink
    May 27, 2011 3:05 am

    Appeal to Popular

    Appeal to popular is that something is good or beneficial because it is popular among people. People often commit this logical error because they naturally think that something is popular would be better than others. It is commonly used in many advertisements. For example, many companies promote their product or service, saying that it is the world’s best selling cosmetics, juice, or television. However, in reality, something is well-known does not necessarily mean it is a good product with better quality. Some can make the sales of their product increased by using a marketing strategy, such as well-designed advertisement or promotion, not because of its own good quality.

    False Analogy

    I want to discuss another logical error: false analogy. I noticed this mistake when I was in high school. At that time, the FTA between Korea and the U.S.A was one of the major controversial issues in Korea. While searching for some advantages and disadvantages of the FTA, I came to visit the website for the Ministry of Foreign Affairs and Trade. As a government part, they agreed with entering into a treaty, saying that it would bring many benefits both of countries. They also added as following, “We already have a bitter historical experience, not having taken timely actions for the future. History repeats itself. Therefore, it is the time to take an opportunity for the bright future and progress.” However, even though something similar happens at a later time, it does not necessarily guarantee that it would have the same result and effect. Despite the fact that something similar happened in the past, the outcome can be totally different, according to a current political, social, or economic situation.

  9. Adam Lee permalink
    May 27, 2011 10:14 pm

    i have ever read a joke about falty logic, it goes:

    tow stranger were talking in a bar.
    A: what’s your job?
    B: technically, i am a logician. i work on logic.
    A: so, what is that?
    B: let me show you an example. do you keep goldfish at home?
    A: yes.
    B: in accordance with the logic, you must put fish in the fish tank or pond, right?
    A: yes, how did you know that? i have a big pond, which has a lot of goldfish in it.
    A: in accordance with common sense, your home has a pond, you should have a large garden.
    A: exactly, I do have a big garden.
    B: from the logical to infer that you have a large pond, is bound to have a big house.
    A: absolutely! i do have a large 5 bedroom house.
    B: according to common sense, one does not build such a big house for himself, if you build a five bedroom house, logically speaking, you should be married.
    A: yes, i’m married, me and my wife and six children live together.
    B: logically speaking, as you have six children, you should have very regular sexual life.
    A: yes, yes, 4 times a week!
    B: well, now you see that, i get to know you and your wife regulally have sex from your goldfish. and this is logic, i do it as my job.
    A: OIC, Thank you.

    then, A’s friend C came around.
    A asked his friend: do you know what is logic?
    C: what is that?
    A: let me show you an example. do you keep goldfish?
    C: i don’t keep fish, why do you ask?
    A: then you and your wife must have no sex!

    did you laugh? at first, i was just laughing at A’s poor logical ability, however, thinking carefully, the logician has logically fault too.
    because everytime A answers the question, he tells the logician more information than simply giving the answer. this is the exact reason how the logician could know the man has regular sexcual life. other wise, no one can infer that from the goldfish.

  10. May 28, 2011 5:07 pm

    Argument from authority

    Aristotle believed that the ideal speed of a terrestrial object is directly proportional to its weight. In other words, when falling down from a high place, a lighter object falls faster than a heavier object.

    Because Aristotle was such a famous Greek philosopher, no one would dare to doubt his idea. People believed his theory about gravity for more than two thousand years, until Galileo proved that the idea was false.

  11. sososo permalink
    May 28, 2011 5:30 pm

    Emphasis Errors

    People make a hasty decision errors by emphasizing certain words.

    1. Do not hurt your friends.
    2. Then we can hurt people who are not our friends.

    The number 1 is the common sense not to hurt other people.
    But some people think like number 2 so they are obsessed with certain words.
    They easily generalize about some special words.

    Hasty Generalization Errors

    People generalize from several specific cases or not enough date.
    Like this proverb ‘See one and know ten’
    As one kind of the mistake or bad behavior, some people are generalized.

    Complex Question Errors

    W : Yesterday You met a woman and have an affair with her?
    M : No, I didn’t cheat.
    W : Anyway, you admit that you met the woman yesterday?

    The women shower him with two questions at the same time
    Even if he denies one question, she will admit the another question is right.

    • proftodd permalink*
      May 30, 2011 9:01 am

      Another similar question is, “Do you still beat your wife?” If you say, “no,” it implies that you used to beat your wife, even though you may never have.

  12. Wow permalink
    May 28, 2011 11:16 pm

    Hasty generalization – Blood type

    Korean and Japanese people tend to believe that all people can be sorted out by blood type. But, according to this logic, there are only 4 types of people all over the world. There are a lot of information and explanation about each character or personality related to blood type. But when I read these types of writting, I realize that each statement suits every people to some degree. So I think sorting out all people only by 4 types can be faulty logic. Do you agree with me????

  13. Book permalink
    May 28, 2011 11:17 pm

    People believe something that is absolute true, if it is said authoritative persons.

    For exmple: Most people completely believe informations that are said through news programs, news paper, books even magazines. But, it may not be true.

  14. Azor. permalink
    May 29, 2011 1:13 pm

    Appeal to heredity

    lots of people might hear about themselves from others like following.

    “You may enter a great university as your father because your father graduated a great university”

    “You will be handsome guy because your parents are good-looking”


    “Why don’t you study hard? If you study hard than before, you can be intelligent like your parents. Because you are given smart brain from your parents”

    This kind of logical faulty is not rare. In addition this will be getting bigger problem named “generalization error”.

    Offspring is not always resemble with his parents.

  15. QIAO MI permalink
    May 29, 2011 3:01 pm

    Yesterday I met a very interesting thing, I have an extracurricular class and at this Saturday we have a test. The extracurricular class in the evening usually and some small tests or additional class are both in the evening. Although the teacher said many times about the test time, I still think the test is in the evening. SO when I go to the classroom no one in here. I call my classmate and asked and I know the exam is in the 9:30AM. I think this experience because a faulty logical.

  16. lalala permalink
    May 29, 2011 9:12 pm

    I think making the mistake of hasty generalization is one of Logical Fallacies.

    (1) Some Koreans eat dog meat
    (2) Some people think that eating dog meat is outrage thing and the person who eat dog meat is hellkite
    (3) Korean is hellkite???

    Explanation: It is true that some Koreans dog meat but not all of us. People around the world think that Korean is hellkite because some Koreans eat dog meat.
    Isn’t it a hasty generalization?

  17. April. Park permalink
    May 29, 2011 9:37 pm

    Here is my example: The Using of “Do”

    One of my acquaintances is very joyful but odd sometimes. One day, he came up with a great idea which was performing as a Santa Claus wearing the costume in the street on Christmas Eve. He was actually very good at performing the Santa, so he thought he was pretty rock himself. After the show, he uploaded his photos to his facebook with this clause “I did SANTA!!”
    All people found him as a pervert because they thought he had a physical relationship with Santa Claus! He had an illogical mistake at using “DO.” The mistake was from his ignorant, however, the people might have considered him as a geek due to his odd character. I thought it was also a part of illogical thinking because they can think it more flexibly.

  18. LJH permalink
    May 29, 2011 9:52 pm

    For example, when someone mistakes at some parts, he or she is valued forever as she or he is failer at that parts. More, he or she is a failer at all parts. But it is a prejudice. people can maistake at some parts, or there is no perfect person. It is possible that someone receive a reprimand beacause of their mistake. But it is not right for them to be valued as a failer forever.
    It is difficult for some celebrities to comeback beacause of just a single mistake. We should not judge someone as we see just one part.

  19. Sohee Kwon permalink
    May 29, 2011 10:06 pm

    Hasty generalization

    There is a saying ‘Don’t judge a book by its cover’
    Hasty generalization is like juding something without deep observation.
    I’m quite familiar with this faulty logic. Here are some examples

    She’s an extravagent person because she is carrying Louis Vuitton bag.
    He was late for today’s meeting so he is surely a lazy person.
    A high school student is smoking over there. Most high school students in this area must be smoking, too.

  20. Choi Hui Young permalink
    May 29, 2011 10:27 pm

    Here is my example : Category Mistake

    Category mistake is a sentence that says of something in one category what can only intelligibly be said of something in another, as when speaking of the mind located in space

    Explanation : A sentence ‘people who don’t like black color don’t like dark color.’ is a good example to explain ‘category mistake. Because, black color is involved in dark color but backward that there is not just black in dark color. Therefore it can assume that people who don’t like black color may like dark brown.

  21. PCE permalink
    May 29, 2011 11:11 pm

    Appeal to popular

    People tend to follow dominant opinion.

    For example, if there is an issue about someone on the internet, people follow the opinion that almost people would say without their subjective viewpoint. It is really dangerous. As a result of this, some people might be hurted and even can die.

  22. Sunny permalink
    May 29, 2011 11:50 pm

    I thought of the logical mistake called “argument to the man”. This is said to attack a person, not contents the person insists.
    For example,
    “the novel the writer wrote is not worth reading because he is the ‘venedict’ who betrayed his country.”

    In this situation, the speaker points to attacking not to the novel’s distirtions of history(for instance,) but to the writer’s a personal situation. This just insults the dignity of the writer.

  23. whosthisgirl permalink
    May 29, 2011 11:51 pm

    Faulty Generalization

    example : Since she has a lot of books at her house, she must be intelligent and gets a good grade at school

    explanation : even if someone has a lot of books at his/her place, that doesn’t mean he/she is intelligent and gets a good grade at school. There are people who have their study (little library thingy) at their house to look cool or some people who like to buy or sellect books and don’t really read or finish to read the books. Also even if some people read books a lot, they might not get a good mark at school. Let’s say I only read novels and hate reading books about science technology, possibly I could get a good mark at literature class but not at science class.

  24. whosthisgirl permalink
    May 29, 2011 11:52 pm

    what the hell! what was I thinking about ? collect* not sellect LOL

  25. kim soo min permalink
    May 29, 2011 11:55 pm

    Judge people by one aspect of them.

    One of my friends tends to judge another person by his rumors. She creates one’s character by all of her judgements. She doesn’t know perfectly about somebody, but she concludes one’s every figure that is shown. For instance, one day, she hears about my rumor that I am a trombenik. And then she makes a hasty conclusion that ‘SM told a lot of lies that she is the smartest student in the class’.
    Its very obvious logical mistake.
    “One fault word can make a tremendous results.”

  26. Youtopia permalink
    May 30, 2011 1:18 am

    Argumentum ad populum(appeal to the people)

    These days, we can easily find this kind of logical fallacy on the Internet, especially on ad.

    For example:
    Brand X vacuum cleaners are the country’s leading brand; so buy Brand X vacuum cleaners.
    Watch Show X – the #1 watched show on television!

    Companies of these products are insisting that you should buy this product because many people are using it!! I also have seen this fallacy at the scandle of ‘Tablo’, the leader of hip hop group Epik High. he had been stigmatized as a diploma forger for most of the year before finally being cleared of the allegations. Many people believed this fact without close investigation because many people claims that this is true. Also, it can be a example of the witch-hunt.

  27. VVVVVera permalink
    May 30, 2011 9:49 am

    1. Perfect Solution Fallacy
    – The perfect solution fallacy is a logical fallacy that occurs when an argument assumes that a perfect solution exists and/or that a solution should be rejected because some part of the problem would still exist after it was implemented. Presumably, assuming no solution is perfect. Then no solution would last very long politically once it had been implemented.
    – For example, one argues that the “terrorist safety net” is a bad idea. Terrorists will still be able to get through.” However, the opponents would argue that some will be still able to get through, but would it be worth stopping those terrorists that it would stop?
    – For example, one argues that anti-drunk driving ad campaigns are not going to work and people are still going to drink and drive no matter what. However, the opponents would argue that it may not eliminate 100% of drunk driving, but it the amount by which it would reduce the total amount of drunk driving enough to make the policy worthwhile?
    2. Slippery Slope Fallacy
    – Slippery slope arguments falsely assume that one thing must lead to another. They begin by suggesting that if we do one thing then that will lead to another, and before we know it we will be doing something that we don’t want to do. They conclude that we therefore shouldn’t do the first thing. The problem with these arguments is that it is possible to do the first thing that they mention without going on to do the other things; restraint is possible.
    – For example, I would like to bring my middle school experience. One of my homeroom teachers in my middle school used to tell us that we should not buy manicure because once we buy manicure, then we would spend more time putting manicure on fingernails. Once you care about your appearance, you will buy everything such as hairpin, bracelets, rings or whatever and before you know it you will forget to study and focus only on your appearance. You don’t want to score a lower grade on exam. You should not buy a manicure. For this reason, I was not allowed to put a manicure on my fingernails for six years in middle and high schools. Ugh what a slippery slope the teachers had fallen for.
    – For example, if you buy a Green Day album, then next you will be buying Buzzcocks albums, and before you know it you will be with green hair and everything. You don’t want to become a punk. You should not buy a Green Day album.

  28. May 30, 2011 9:49 am

    Many reporters usually use a faulty generalization in the news’ headline to gather people’s attraction. For example, lately, the well known Korean announcer Song Jiseon committed suicide after suffering from depression. However, lots of news and people said her death was caused by the SNS, social network service such as tweets, face book, because there were many online users’ attacks and insults on the her site. Even though the SNS could be one of causes for her death, many people overlooked other causes such as depression, scandals. So, when reading articles, we should try to see the hidden side and to avoid sensational contents.

  29. Sylpid permalink
    May 30, 2011 9:51 am

    Argumentum and Ignorantiam

    People often insist some fact true because it is not proved to be not true.
    Here is an example.
    A: Does god really exist?
    B: Are you sure that god does not exist?
    A: No.
    B: So god really exist.

  30. terry permalink
    May 30, 2011 3:26 pm

    In my case I am useally used to concusing my causes and dffects.
    If my bag strap was borken, I always think that i will meet with something bad happening such as breaking up with boyfreind.
    It’s rediculous thinkg. BUT this superstitions widespread in our life.

  31. S.A. Lee permalink
    May 30, 2011 4:33 pm

    Division Errors
    a fact of a whole thing shuld be the fact of the part.

    A : U.S.A. is the biggestin economy the world of this year.
    B : Really?! Then, every american is a rich, isn’t it?

    Complex Question

    A: Did you steal the momey and spend all?
    B: No, i didn’t
    A: Okay then, you admit that you stole the money, aren’t you?

    A asks two questions at once. If B denied one question, A would accept the other one’s answering as ‘yes’.

    • S.A. Lee permalink
      May 30, 2011 4:34 pm

      biggestin economy the world
      -> biggest ecomony in the world.

  32. Hyeung-Ha permalink
    May 31, 2011 12:35 am

    [false analogy]

    My example-

    The Berlin Wall was set up between West Berlin and East Germany in order to have a symbolic boundary between Communism and Democracy.

    This is the case with South and North Korea and obviously still being at the stage of war, there exists the DMZ between those two boarders. Since the fall of the Berlin Wall in 1989, which paved the way for the split Germans at the time to a reunification also converting to democracy we can bet that both Korea’s can reunifiy if they follow their footsteps and did exactly what West Berlin and East Germany did.

    Explanation: Even though the Berlin Wall fell in 1989, we cannot assume that North and South Korea will bring down the DMZ one day. Due to the fact
    their situation is complicated, and there exists many aspects of cons such as losing economic power(If they reunify Korea will lose its stronghold in the world because it’ll have to make repaires at home)
    In the 21st century globalization period, versues the Cold war day, everything is more focused on economic and financial aspects nowadays.

  33. soyoung L permalink
    June 1, 2011 8:58 am

    OMG!! It is exactelly my sectences…. I should have read this page before I wore my essay. It has much more information than I expected and well organized, so I can use and studay easily.

  34. Hyo Yeol Kim permalink
    June 1, 2011 11:05 am

    For example)
    A- Cosmetic’s Saleswoman
    B- Consumer

    A: Come in, this products is new.
    If you will use this, you have more pure face skin than now.
    B: This is ridiculous. so you have pimples on you face.

    Comment : Consumer think because saleswoman has pimples, this products could not make my face more purely than now. Consumer judges based on saleswoman’s appearance. But saleswoman may have pimples for others reasons.

  35. Da Jung Lee permalink
    June 2, 2011 3:51 pm

    I investigate some kind of logical mistakes.
    Argumentum Ad Ignorantiam
    Argumentun Ad Verecundiam
    Argumentum Ad Hominem
    Argumentum Ad Populum
    Argumentum Ad Misericordian
    Argumentum Ad Baculum
    Ignoratio Elenchi
    ..and so on

    and This is my example.

    If someone have mustache, he is man.
    Tom doesn’t have mustache. so Tom is not man.

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