Logic and Logical Fallacies

1. Logic and Logical Fallacies Critical Thinking!!

2. Induction • Most common type of argument • Can never be 100% sure that it is true because a new piece of evidence might come along • Conclusions are based on many pieces of evidence (data or facts or statistics) • Involves the inductive leap to a conclusion • Is a feature of the scientific method

3. Deduction and deductive reasoning • You can be sure that correct deductive reasoning is guaranteed to be true • Begins with a premise that is usually scientific or legal (like the U.S. Constitution) – something which everybody agrees is true • Argues that a particular case “fits” the premise

4. Syllogism is the form for deductive reasoning • Major premise (All people have hearts) • Minor premise (Jeremy is a person) • _______________________________ • Conclusion (Jeremy has a heart)

5. What makes Induction wrong? • Not enough evidence (jumping to conclusions or hasty generalization) • Evidence must be true; statistics, for example, must be correct. The MATH must be correct.

6. What makes Deduction wrong? • Major premise is wrong (All teachers love coffee) • Minor premise doesn’t fit the case (My mother loves tea.) • It’s not set up to cross-cancel

7. Toulmin Logic • Looks just like a syllogism in deductive reasoning except upside down • Conclusion is first (Claim) • Minor premise next (Data) • Major premise (Warrant) • Also, you normally use qualifying words like “might” or “possibly”

8. Post hoc fallacy (false cause) • One assumes that because one event happened after another event, the earlier event caused the later one. • Example: Because I didn’t wear my lucky shirt, I didn’t get an A on the midterm • Example: Steve Jobs refused to give any money to De Anza College; then he died. Better give money to De Anza!!

9. Superstitions are one type of Post hoc fallacy • I left my umbrella at home; therefore it rained. • I broke a mirror; now I just got a ticket (bad luck).

10. False analogy • Comparison in which a surface similarity masks a significant difference • Example: We know that we can’t ban cigarettes because the Prohibition period (when alcohol was banned) just led to more crime • We lost the war in Vietnam because the U.S. can’t fight countries having civil wars; therefore we will lose in Iraq and in Afghanistan.

11. Hasty generalization • By far the most common logical fallacy • Not enough evidence to support a conclusion • Example: Every paper I saw showed an A on the midterm, so everybody in the class must have gotten A’s.

12. Slippery slope • We assume that one small action will trigger a huge negative result • If you don’t pass the next quiz, then you’ll flunk the course, and then you’ll flunk out of De Anza and then you’ll never get rich.

13. Bandwagon • An argument that depends on going along with the crowd on the false assumption that a large group of people will know the truth. • Since everybody I know wants to go to Berkeley, it must be the school for me.

14. Ad hominem • Personal attack on someone who disagrees with you, used instead of actually countering that person’s argument • My opponent can’t be trusted to be governor because he she has been divorced • My opponent has changed her hair style five times this month; will she change her opinions just as often?

15. Either or fallacy • Reduces a complicated decision to only two options • It’s me or the dog. Which do you want? • You can either work at Great America or you can take math during summer school. Those are your choices.

16. False authority • This logical fallacy occurs when a person (usually a famous person) endorses a product. If the celebrity has no expertise on the product, then the ad is guilty of “false authority.”

17. _______________? I just can’t decide between being an optometrist and being an accountant. I just go back and forth and see the advantages and disadvantages of each. Here I am, 18 years old, and don’t even know what my career is going to be.

18. ________________? I washed my car last week and then it rained, so it’s clear that washing one’s car causes it to rain.

19. __________________? • Everybody in our class is going to the Asian-American comedy club Friday night, so you should, too.

20. ________________? • Ms. Patton shows a film clip almost every class, so she must be an expert on film.

21. _________________? I know that in the US, for New Year’s people have parties, drink too much, and kiss somebody at midnight, so Chinese and Vietnamese New Year’s must be the same.

22. _______________? • The automotive equivalent of a really hot librarian.

23. _______________? • I know I can quit smoking. No problem. As soon as I figured out that eating fries was making me gain weight, I quit and lost 5 pounds. So I’ll just do it with smoking.

24. ___________________? I have a photo of President Obama shaking hands with President Mubarak of Egypt, so President Obama must be in favor of a repressive government, too.

25. ______________? Every time my daughter throws a ball with her right hand, she can hit the target. However, every time she throws with her left hand, she misses. She must be right-handed. This is NOT a logical fallacy. It’s an example of one type of reasoning.

26. ____________________? • According to the Constitution, people who live in the U.S. have freedom of speech. • Andy lives in the U.S. • Conclusion: Andy has freedom of speech. • This is not a logical fallacy. It is a type of logical reasoning.

27. ___________? Beyonce is so hot when she advertises this great new L’Oreal lipstick, so I’m going to buy it because if she likes it, it must be fabulous.

28. _________________? First you throw your recyclable can in the trash, then you take a fifteen-minute shower, and before you know it, the North and South Pole ice has melted and Foster City and Redwood City are under water.

29. __________________? • The first four weeks of Calculus have been easy, so I am sure that I’m going to get an A.

30. __________________? First, you’ll go out on one date with a white person and then you’ll fall in love and before you know it, you’ll have Hapa babies!

31. ______________________? People in our class are going to UCLA or Cal. They must be really smart.

32. ______________? Students in our class are creative. Jasmine is in our class __________________________ Jasmine is creative. The above is not a logical fallacy; it is a ________.

33. ____________? Rodney: This doctor. Is he good? Thick: Yup. Best in the business. He even worked on Schwarzenegger.