The document discusses reasoning and argumentation. It provides examples of valid and invalid logical structures like syllogisms. It also discusses the differences between deductive and inductive reasoning. The document outlines common fallacies of reasoning and why people may use faulty logic, such as ignorance, laziness, pride, and prejudice. Overall, the document examines how to evaluate arguments and outlines some limitations of reason in establishing absolute certainty.

1. Reason

2. 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?

3. 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

4. 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:

5. 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

6. 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

7. 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 valid
all As are Bs
some As are Cs
therefore some Bs are Cs
A C
B
Your premises don’t have to
be true to still make a logical
argument

8. 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”

9. 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.

10. 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

11. 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

12. 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 heated
Metal B expands when heated
Therefore all metals expand when
heated
More certainty
Less informative
Less certainty
More informative
Deduction and Induction

13. 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

14. 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

16. 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?

17. 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

18. 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

19. 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

20. 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

21. 3
• The ends justify the means. After all, you
have to break eggs to make an omelette
Post hoc ergo
propter hoc

22. 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

23. 5
• That can’t be true, Francisco is really
clever and he doesn’t believe it
Ad Hominem

24. 6
• We got on really well on both dates we’ve
been on. We must be well suited. Let’s get
married.
Hasty
generalisation

25. 7
• If you’re not part of the solution then you’re
part of the problem.
False dilemma

26. 8
• Since Isaac Newton believed in God, there
must be some truth in religion
Ad Hominem

27. 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

28. 10
• Nobody has proved that we are alone in
the Universe, therefore aliens must exist.
Argument ad
ignorantium

29. 11
• Is your whole family stupid, or is it just
you?
Loaded Question

30. 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

31. 13
• Nobody succeeds without hard work.
Since you failed your exams, it shows how
lazy you have been
False Analogy

32. 14
• She has never been involved in any kind
of scandal, therefore she must be a very
honest person.
Post hoc ergo
propter hoc

33. 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

34. 16
• If you are against the war on terror then
you are against the United States
– George W Bush
False dilemma
False dichotomy

35. 17
• The English can’t cook. He’s English
therefore he can’t cook
Hasty
generalisation

36. 18
• I deserve to be given the job as I have
seven children and a sick mother to look
after.
Ad Misericordiam
(special pleading)

37. 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

38. Appeal to nature
20

39. Circular reasoning
21

40. Invalid syllogism
22

41. 23
Slippery slope

42. False analogy

43. Straw man
argument

44. 24
Non-sequitur

45. 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

46. 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