Reason

Introduction• There is a scene in a Broadway play where a priest is

speaking to a man at a party and is asked if he ever hears anything interesting in confession. The priest replies that in fact that in the very first confession he gave after being ordained a man admitted that he’d murdered his wife

• Later on in the party another man is introduced to the

priest and says “I met you a long time ago father, in fact I was the first person who came to you for confession”

• What does the author expect you to conclude from the two previous conversations?

The structure of arguments of reason

• All ostriches are birds• Carlos is an ostrich• Carlos is a bird

Premise 1

Premise 2

Conclusion

This is a syllogism – 2 premises leading to a conclusion

Sherlock Holmes

• Men with chalk dust on their fingers are schoolteachers

• This man has chalk dust on his fingers • This man is a schoolteacher

In one story, Holmes is observing a man sitting opposite him on a train. Here is his train of thought:

Sherlock Holmes

• All watchdogs bark at strangers• The watchdog did not bark when the

house was being burgled• Therefore the burglar was not a stranger

The structure of arguments of reason

• All ostriches have feathers • A turkey has feathers• All turkeys are ostriches

An invalid syllogism occurs where the conclusion is false even if the premises are true

The structure of arguments of reason

• All fimbles are foobles • Some fimbles are wombles• Some foobles are therefore wombles

Arguments of this type are always validall As are Bssome As are Cstherefore some Bs are Cs

A C

B

Your premises don’t have to be true to still make a logical argument

Example of flawed logic

In a campaign speech during the summer of 1952, Senator Joseph McCarthy, who had made a reputation as a tireless enemy of communism, said, "I do not tell you that Schlesinger, Stevenson's number one man, number one brain trust,  I don't tell you he's a Communist.  I have no information on that point.  But I do know that if he were a Communist he would also ridicule religion as Schlesinger has done. ”

“Guilt by association”

Test Your Reasoning Skills

The old man had just turned off the lights in the shop and was preparing to lock up when a youth appeared and demanded money. The owner opened the cash register; the contents were grabbed, and the man ran away. The police were informed immediately.

Test Your Reasoning Skills• Are the following statements: true, false, or is there not enough information to come to a

conclusion?

1. A young man appeared after the lights had been turned off

2. The old man was preparing to go home

3. The robber demanded money

4. Someone opened the cash register

5. The robber demanded money from the owner

6. The person who opened the cash register was a man

7. The cash register contained money, but we are not told how much

8. The gender of the owner is not revealed in the story

9. The robber did not demand money

10.After the young man grabbed the contents of the cash register he ran away

11. A young man appeared after the lights had been turned off

12.The robber was a man

13.The owner was a man

14.The owner appeared and demanded money

15.The man ran away after he had demanded money

Questions

• Make up syllogisms for the following:1. Two true premises and a true conclusion

2. One true premise, one false premise and a true conclusion

3. One true premise, one false premise and a false conclusion

4. Two false premises and a true conclusion

5. Two false premises and a false conclusion

Note that just because a syllogism is valid, it doesn’t mean that the premises or the conclusion are true

DEDUCTION INDUCTION

Goes from the general to the particular

Goes from the particular to the general

All metals expand when they are heated

Therefore this metal will expand when I heat it

Metal A expands when heatedMetal B expands when heated

Therefore all metals expand when heated

More certainty

Less informative

Less certainty

More informative

Deduction and Induction

The Problem of Deduction

• Deduction appears to be better as it has more certainty associated with it

• However, the original premise of a deduction must be based to some extent on INDUCTION

• For example, Sherlock Holmes’ deductive premise that all watchdogs bark at strangers must have been based on his inductive observation that dogs A, B, C and D… don’t bark at strangers (he can’t have observed the set of ALL watchdogs)

In the books Sherlock Holmes famously used his ‘powers of deduction’, however he usually reasoned using a mixture of deduction and induction

The Problem of Induction

• The problem of induction is more obvious

• However strong your premise appears to be, it only takes one exception to come along and ruin everything

• For example Europeans once thought that all swans are white, based on the inductive premise that every swan they’d ever seen in Europe was white…. They were wrong

Induction and Deduction

• Larger animals have slower heart rates... Do we conclude this from the data by deduction or induction?

• Estimate the heart rate of an Etruscan pygmy shrew which weighs only 1.3 grams. Did you reason by deduction or induction?

What makes some Inductions Better than Others?

• Number of observations

• Variety of observations

• Attempt to search for exceptions

• Coherence – “Extraordinary claims require extraordinary evidence” - Carl Sagan

• Subject area – e.g. generalisations are more reliable in the natural sciences than the social sciences

Fallacies of Reason

• A fallacy is an argument that uses faulty reasoning

• A formal fallacy is reasoning that is always wrong due to a flaw in logical structure – it is invalid in form

• An informal fallacy is reasoning that is wrong due to one or more false premises – it is valid in form

http://edrontheoryofknowledge.blogspot.mx/2010/03/the-logical-fallacy-song.html

1

• Since strict gun controls were imposed in Mexico City, the crime rate has risen. This shows that gun control does not reduce crime.

Hasty Generalisation

2

• She says that I’m her best friend, and it must be true, because she wouldn’t lie to her best friend.

Circular reasoning

3

• The ends justify the means. After all, you have to break eggs to make an omelette

Post hoc ergo propter hoc

4

• Since Juan always talks about science, you can be sure that if you meet somebody who’s interested in science they must be related to Juan.

Hasty Generalisation

5

• That can’t be true, Francisco is really clever and he doesn’t believe it

Ad Hominem

6

• We got on really well on both dates we’ve been on. We must be well suited. Let’s get married.

Hasty generalisation

7

• If you’re not part of the solution then you’re part of the problem.

False dilemma

8

• Since Isaac Newton believed in God, there must be some truth in religion

Ad Hominem

9

• The average family in Mexico has 3 children. The Hernandez’s are a very average family, therefore they must have 3 children.

False Analogy

10

• Nobody has proved that we are alone in the Universe, therefore aliens must exist.

Argument ad ignorantium

11

• Is your whole family stupid, or is it just you?

Loaded Question

12

• No great artist has been recognised during his lifetime. My work has not been recognised, therefore I must be a great artist

Post hoc ergo propter hoc

13

• Nobody succeeds without hard work. Since you failed your exams, it shows how lazy you have been

False Analogy

14

• She has never been involved in any kind of scandal, therefore she must be a very honest person.

Post hoc ergo propter hoc

15

• A car owner is more likely to look after her own car than one she rents. Therefore it stands to reason that a slave owner will look after his slaves better than an employer looks after his workers.

Post hoc ergo propter hoc

16

• If you are against the war on terror then you are against the United States– George W Bush

False dilemma

False dichotomy

17

• The English can’t cook. He’s English therefore he can’t cook

Hasty generalisation

18

• I deserve to be given the job as I have seven children and a sick mother to look after.

Ad Misericordiam (special pleading)

19

• "We are constantly told that Mexican pork is not dangerous. But at the same time, nobody has proved that it is safe." - Nikolai Vlasov, Russian chief veterinary

inspector

Argument ad ignorantium

Appeal to nature

20

Circular reasoning

21

Invalid syllogism

22

23

Slippery slope

False analogy

Straw man argument

24

Non-sequitur

Bad Reasoning

• Ignorance – failure to spot our own fallacies• Laziness – not being bothered to check the

evidence• Pride – we don’t like to think of ourselves as

being closed-minded, and we all want to win an argument once we are in one

• Prejudice – The way we think about things is always linked to our pre-existing ideas on the subject (being open-minded is hard)

But remember – “Be open-minded, but not so open-minded that your brains fall out” Carl Sagan

Reason and Certainty• The existence of fallacies suggests that reason

(and logic) does not provide certainty• The fact that even simple syllogisms can lead to

false conclusions suggests that true conclusions can only be drawn if you begin with the truth

• At least we can say that logical thinking provides a clear framework to get to true conclusions

• Some people have such a warped sense of logic (through ignorance, laziness, pride or prejudice) that they perhaps can’t be reasoned with

Bananas – the atheists nightmare