What is a logical fallacy?

Logical fallacies

Do you know what a “fallacy” is? Look at the word – it has “falla” in it, which could mean “fault,” “flaw,” or “fail” So, a fallacy is: –a flaw in how you reason

The more churches there are in a city, the more prostitutes there are.

What conclusion could you draw from this correlation? Would you conclude that church causes women to go into prostitution? Wrong! This is a logical fallacy!

Connor says that he knows someone who is 90 and who has smoked 40 cigarettes a day for 75 years and he’s OK, so smoking can’t be harmful.

Eleanor bought a good luck charm and he seems to be doing better at school.

Now, you will see some specific types of logical fallacies. Choose one or two that you think are the most interesting.

A summary of common fallacies

Post hoc ergo propter hoc Assuming one thing causes another just because one follows another. “Theory of Knowledge for the IB Diploma”, Richard van de Lagemaat, Cambridge University Press

Ad ignorantiam Claiming that something is true because it cannot be proved to be false. “Theory of Knowledge for the IB Diploma”, Richard van de Lagemaat, Cambridge University Press

Hasty generalisation Generalising from insufficient evidence. “Theory of Knowledge for the IB Diploma”, Richard van de Lagemaat, Cambridge University Press

Ad hominem Attacking /supporting the person rather than the argument. “Theory of Knowledge for the IB Diploma”, Richard van de Lagemaat, Cambridge University Press

Circular reasoning Assuming the truth of what you are supposed to be proving. “Theory of Knowledge for the IB Diploma”, Richard van de Lagemaat, Cambridge University Press

Special pleading Using double standards to excuse an individual or group. “Theory of Knowledge for the IB Diploma”, Richard van de Lagemaat, Cambridge University Press

Equivocation Using language ambiguously (when a word has two meanings). “Theory of Knowledge for the IB Diploma”, Richard van de Lagemaat, Cambridge University Press A feather is light. What is light cannot be dark. Therefore, a feather cannot be dark.

False analogy Assuming that because two things are alike in one way that they are alike in other respects. “Theory of Knowledge for the IB Diploma”, Richard van de Lagemaat, Cambridge University Press

False dilemma Assuming that only a limited number of alternatives exist. “Theory of Knowledge for the IB Diploma”, Richard van de Lagemaat, Cambridge University Press You are either ALL GOODor ALL BAD

Loaded question A question that is biased because it contains a built-in assumption. “Theory of Knowledge for the IB Diploma”, Richard van de Lagemaat, Cambridge University Press Have you stopped cheating on your wife?

In each of the following cases, decide which best applies to the argument. Hasty generalisation, post hoc ergo propter hoc, circular reasoning, ad hominem, special pleading, ad ignorantium, false dilemma, false analogy, equivocation, loaded question.

1. Since strict gun control laws were introduced in Dodge City, the crime rate has risen. This shows that gun control does nothing to reduce crime. “Theory of Knowledge for the IB Diploma”, Richard van de Lagemaat, Cambridge University Press Hasty generalisation, post hoc ergo propter hoc, circular reasoning, ad hominem, special pleading, ad ignorantium, false dilemma, false analogy, equivocation, loaded question.

post hoc ergo propter hoc Since strict gun control laws were introduced in Dodge City, the crime rate has risen. This shows that gun control does nothing to reduce crime.

2. Emmeline said she trusted me, and she must be telling the truth because she wouldn’t lie to someone that she trusted. “Theory of Knowledge for the IB Diploma”, Richard van de Lagemaat, Cambridge University Press Hasty generalisation, post hoc ergo propter hoc, circular reasoning, ad hominem, special pleading, ad ignorantium, false dilemma, false analogy, equivocation, loaded question.

circular reasoning Emmeline said she trusted me, and she must be telling the truth because she wouldn’t lie to someone that she trusted. “Theory of Knowledge for the IB Diploma”, Richard van de Lagemaat, Cambridge University Press

3. The ends justifies the means. After all, if you want to make omelettes, you have to break eggs. “Theory of Knowledge for the IB Diploma”, Richard van de Lagemaat, Cambridge University Press Hasty generalisation, post hoc ergo propter hoc, circular reasoning, ad hominem, special pleading, ad ignorantium, false dilemma, false analogy, equivocation, loaded question.

false analogy The ends justifies the means. After all, if you want to make omelettes, you have to break eggs.

4. That can’t be right, non of my friends would believe it. “Theory of Knowledge for the IB Diploma”, Richard van de Lagemaat, Cambridge University Press Hasty generalisation, post hoc ergo propter hoc, circular reasoning, ad hominem, special pleading, ad ignorantium, false dilemma, false analogy, equivocation, loaded question.

ad hominem That can’t be right, non of my friends would believe it.

5. Since many great scientists have believed in God, there must be some truth in religion. “Theory of Knowledge for the IB Diploma”, Richard van de Lagemaat, Cambridge University Press Hasty generalisation, post hoc ergo propter hoc, circular reasoning, ad hominem, special pleading, ad ignorantium, false dilemma, false analogy, equivocation, loaded question.

ad hominem Since many great scientists have believed in God, there must be some truth in religion.

6. Teddy and Seanne got on very well on their two dates together. They are clearly well suited and should get married. “Theory of Knowledge for the IB Diploma”, Richard van de Lagemaat, Cambridge University Press Hasty generalisation, post hoc ergo propter hoc, circular reasoning, ad hominem, special pleading, ad ignorantium, false dilemma, false analogy, equivocation, loaded question.

hasty generalisation Teddy and Seanne got on very well on their two dates together. They are clearly well suited and should get married.

7. Do you want to be part of the solution, or part of the problem? Hasty generalisation, post hoc ergo propter hoc, circular reasoning, ad hominem, special pleading, ad ignorantium, false dilemma, false analogy, equivocation, loaded question. “Theory of Knowledge for the IB Diploma”, Richard van de Lagemaat, Cambridge University Press

false dilemma Do you want to be part of the solution, or part of the problem?

8. “I agree that you shouldn’t copy mp3s illegally, but I’ve only copied a few and I do buy tracks sometimes.” “Theory of Knowledge for the IB Diploma”, Richard van de Lagemaat, Cambridge University Press Hasty generalisation, post hoc ergo propter hoc, circular reasoning, ad hominem, special pleading, ad ignorantium, false dilemma, false analogy, equivocation, loaded question.

special pleading “I agree that you shouldn’t copy mp3s illegally, but I’ve only copied a few and I do buy tracks sometimes.”

9. The average UK family has 2.5 children. The Smiths are average people, they must have 2.5 kids. “Theory of Knowledge for the IB Diploma”, Richard van de Lagemaat, Cambridge University Press Hasty generalisation, post hoc ergo propter hoc, circular reasoning, ad hominem, special pleading, ad ignorantium, false dilemma, false analogy, equivocation, loaded question.

equivocation The average UK family has 2.5 children. The Smiths are average people, they must have 2.5 kids.

10. Since no-one has been able to prove that God didn’t create the universe, we must conclude that God did make everything. “Theory of Knowledge for the IB Diploma”, Richard van de Lagemaat, Cambridge University Press Hasty generalisation, post hoc ergo propter hoc, circular reasoning, ad hominem, special pleading, ad ignorantium, false dilemma, false analogy, equivocation, loaded question.

ad ignorantium Since no-one has been able to prove that God didn’t create the universe, we must conclude that God did make everything.

11. Are all your family stupid, or is it just you? “Theory of Knowledge for the IB Diploma”, Richard van de Lagemaat, Cambridge University Press Hasty generalisation, post hoc ergo propter hoc, circular reasoning, ad hominem, special pleading, ad ignorantium, false dilemma, false analogy, equivocation, loaded question.

loaded question Are all your family stupid, or is it just you?

12. No scandal has ever touched Mr Porter. He must be a clean living and honest man. “Theory of Knowledge for the IB Diploma”, Richard van de Lagemaat, Cambridge University Press Hasty generalisation, post hoc ergo propter hoc, circular reasoning, ad hominem, special pleading, ad ignorantium, false dilemma, false analogy, equivocation, loaded question.

ad ignorantium No scandal has ever touched Mr Porter. He must be a clean living and honest man.

13. Just as you are more likely to take care of a car that you own rather than a car that you rent, a slave owner would look after a slave better than an employer would look after their worker. “Theory of Knowledge for the IB Diploma”, Richard van de Lagemaat, Cambridge University Press Hasty generalisation, post hoc ergo propter hoc, circular reasoning, ad hominem, special pleading, ad ignorantium, false dilemma, false analogy, equivocation, loaded question.

false analogy Just as you are more likely to take care of a car that you own rather than a car that you rent, a slave owner would look after a slave better than an employer would look after their worker.

14. To ignore the possibility that Norway landed the first person on the moon just because nobody has heard of a Norwegian space program is arrogant. If we are unaware of something does not mean that it never happened. “Theory of Knowledge for the IB Diploma”, Richard van de Lagemaat, Cambridge University Press Hasty generalisation, post hoc ergo propter hoc, circular reasoning, ad hominem, special pleading, ad ignorantium, false dilemma, false analogy, equivocation, loaded question.

ad ignorantium To ignore the possibility that Norway landed the first person on the moon just because nobody has heard of a Norwegian space program is arrogant. If we are unaware of something does not mean that it never happened.

15. In the fight against terrorism, you are either with the USA or against. “Theory of Knowledge for the IB Diploma”, Richard van de Lagemaat, Cambridge University Press Hasty generalisation, post hoc ergo propter hoc, circular reasoning, ad hominem, special pleading, ad ignorantium, false dilemma, false analogy, equivocation, loaded question.

false dilemma In the fight against terrorism, you are either with the USA or against.

Definition Try-Out I Know Neil Armstrong landed on the moon in I know that strawberries are red. abb cacI know that if a is bigger than b, and b is bigger than c, then a is bigger than c. I know that murder is wrong. your own Try these examples against each definition of knowledge : Which ones work best with which way of knowing?

What is your definition of Knowledge? Questions and grey areas remain about: the role of judgment; the need for evidence, and the reliability of that evidence; the role of skepticism, and the dangers of too much; the need for belief… People who believe absurdities will commit atrocities Voltaire