2. The attraction of reason
Reason seems to give us certainty
Reason appears to provide justification for claims
Reason allows us to extract new knowledge from observation and
other knowledge
3. Principle types of reasoning
Deductive reasoning
◦ Given the truth of some information, the conclusion must also be
true
◦ A All humans are mortal
◦ B I am human
◦ C Therefore I am mortal
Inductive reasoning
◦ Where observation leads to conclusions
◦ All swans we have ever seen are white, so all swans must be white
4. Deductive reasoning
Moves from the general to the particular
A. All cats are mammals
B. Garfield is a cat
C. Therefore Garfield is a mammal
This is called a syllogism though a syllogism can be valid without
being true…
(what if Garfield was actually a dog or a cartoon character?)
6. Validity
a p r e m i s e
r
TRUE FALSE
g
VALID Conclusion
u
= True
m
e
n
t
Conclusion
= T or F
INVALID Conclusion
= T or F
Conclusion
= T or F
All cats are mammals.
Therefore Jane’s cat
Tabitha is a mammal.
All cats are black.
Therefore Jane's cat
Tabitha is black.
All rich women have
diamond necklaces.
My aunt has a
diamond necklace so
she must be rich.
I have never seen a black
rose. Therefore there are
no black roses.
7. Inductive reasoning
◦ From the particular to the general
◦ Observation leads to the discovery of a pattern
◦ This allows a hypothesis or prediction to be made
◦ This leads to a general theory about how things are/work
Metal A expands when heated; metal B expands when heated; metal C expands when
heated........... When I next heat a metal ....... It will expand because all metals expand
when heated.
All swans I have ever seen are white....... When I next see a swan....... It will be white (all
swans are white)
I am always depressed in winter, so next winter ....... I will be depressed (winter is
depressing)
8. Real life– bad reasoning
Ad Misericordiam- Appeal for sympathy
We hope you will accept our recommendations. We have spent 12 weeks
working overtime on them and we are quite exhausted
Appeal to authority
One of the world’s top economists says that interest rates will rise next year, so it
must be true.
Focusing on the arguer and not the argument (ad hominem)
Since the honourable gentleman has only just admitted that he failed to declare
all the donations made to his campaign, his plans for revising the law on
charitable giving must be seriously flawed.”
9. Bad reasoning (2)
Circular reasoning
“Why do you have the most pearls?” asked one of the pirates.
“Because I am the leader.”
“Why are you the leader?”
“Because I have the most pearls.” (Anthony Flew)
Special pleading
I should be allowed to go to the front of the lunch queue because I have to work
through lunchtime to finish my coursework which I couldn’t do at the weekend
and it has to be handed in at the start of my lesson this afternoon.
10. Bad reasoning (3)
Equivocation
Fast food is better than nothing, but nothing is better than staying healthy. So
fast food is better than staying healthy.
Argument from ignorance (ad ignorantiam)
“There is nothing in his records to disprove his communist connections.”
(Senator Joe McCarthy)
False analogy
The ends justify the means. After all if you want to make an omelette you have
to break some eggs.
11. Bad reasoning (4)
False dilemma
Do those who suggest an increase in military spending really want to see our
schools and hospitals run down?
Hasty generalisation
All the experiments in our Chemistry class showed that tap water is as acidic as
dilute hydrochloric acid. We should inform the water company immediately!
12. Emotion & Reason
Emotion Reason
Furious Solving a maths
Is there a link between emotion and reason?
Can emotions be rational?
problem
Getting
annoyed
13. Reason & Emotion – some common
stereotypes
◦Heart v. head
◦Warm (even hot) v. cold
◦Passionate v. rational
◦Interesting v. dull and boring
◦Insight v. deduction
◦Unreliable v. reliable
◦Illogical v. logical
◦Uncertain v. certain
14. Emotion – inferior to reason?
“The heart has its reasons which reason cannot know” Blaise Pascal
“Reason is, and ought only to be the slave of the passions, and can never
pretend to any other office than to serve and obey them” David Hume
“Brain scans show us that the [emotions]act faster and often bypass the
[rational]. Not the other way round. Signals are sorted for meaning faster than
the conscious mind can register” Goleman
15. Bertrand Russell
“The opinions that are held with passion are always those for which no good
ground exists; indeed the passion is the measure of the holder’s lack of
rational conviction. Opinions in politics and religion are almost always held
passionately.”
“All his instincts were on the side of the "rationalists"; his greatest hatred
was for those who exalted emotion, or any sort of mystic intuition, at the
expense of reason. But because Russell was the greatest rationalist of all, he
had to admit that reason cannot prove the mystics wrong.” AlanWood
16. What do we know through emotion?
1) In Literature and the Arts?
2) In science or mathematics
3) About the past?
4) About other cultures?
5) In making judgements...
◦ About right and wrong?
◦ About what to do?
◦ About other people?
17. Some questions about emotion
a) Can we know anything through emotion alone?
b) Can emotion ever provide any certainty?
c) Should we always set aside emotion when it comes to knowledge?
d) Would you always trust a knowledge claim where the person
making it makes an appeal to emotion?
18. Try asking the same questions about
reason!
a) Can we know anything through reason alone?
b) Can reason ever provide any certainty?
c) Should we always set aside reason when it comes to knowledge?
d) Would you always trust a knowledge claim where the person
making it makes an appeal to reason?
