ToK - Reason

1. Reason (noun)a basis or cause, as for some belief, action, fact,

event, etc

2. Reason (verb)- to think or argue in a logical manner;- to form conclusions, judgments, or inferences

from facts or premises

ToK - Reason

“He that will not reason is a bigot;he that cannot reason is a fool;

and he that dares not reason is a slave”William Drummond 1585-1649

ToK - Reason

The curious incident of the dog in the night ....

The path is wet in the morning ....

You left your mobile phone either on your desk or in your coat pocket ...

ToK - Reason

• Deductive reasoning

• Inductive reasoning

• Informal reasoning

ToK - Reason

Deductive reasoningFrom general to particular

• All humans are mortal• Socrates is human• Therefore Socrates is mortal

• Two premises and a conclusion• Three terms – each occurring twice• Quantifiers

(This is formally called a Syllogism)

ToK - Reason

1. Argument (noun) - an oral disagreement; verbal opposition;

contention; altercation: a violent argument

2. Argument (noun)- a process of reasoning; series of reasons: I

couldn't follow his argument.

ToK - Reason

Truth and Validity• Truth: concerned with what is the case• Validity: only concerned with whether

conclusions follow from premises

• You should not say that an argument is true or false, but that it is valid or invalid

ToK - Reason

Inductive reasoning

From the particular to the general (the observed to the unobserved)

• I have never heard of a human being who didn’t die ... then we generalise to ...

• All human beings are mortal

IR allows us to make generalisations about the world.

ToK - Reason

Will the sun rise tomorrow?

How do you know? How certain are you?

ToK - Reason

Science uses inductive reasoning:

Formulates general laws on the basis of limited observations -

• Metal A expands when heated• Metal B expands when heated• Metal C expands when heated• ..... a conclusion?

ToK - Reason

• Give examples of some hasty generalisations

• Why do you think that people are so quick to jump to conclusions?

• What is the difference between a prejudice, a generalisation and a scientific law?

ToK - Reason

What distinguishes good generalisations?

• Number – you see one dog swimming

• Variety – different circumstances; old, young;

• Exceptions – find counter-examples

• Coherence – you should demand more evidence to support surprising claims than unsurprising ones

• Subject area – eg. generalisations tend to be more reliable in the natural sciences than in the social sciences

ToK - Reason

Informal reasoning – Fallacies (Invalid patterns of reasoning)

• Post hoc ergo propter hoc – assume that because B follows A that A must be the cause of B

• Ad hominem fallacy – attack or support the person rather than the argument• Circular reasoning – assume the truth of something you are supposed to be

proving• Special pleading – making an exception in your own case that you would not

find acceptable if it came from someone else• Equivocation – using language ambiguously• Argument ad ignorantium – claim that something is true on the grounds that

there is no evidence to disprove it• False analogy – assume that because two things are similar in some ways

that they must be similar in some further way• False dilemma – assume that only two alternatives exist when there are in

fact a wide range of options• Loaded questions – a question that is biased because it contains a built-in

assumption

ToK - Reason

• Since strict gun controls were introduced in Dodge City, the crime rate has risen. This shows that gun control does nothing to reduce crime.

• Jane said she trusted me, and she must be telling the truth because she wouldn’t lie to someone that she trusted.

• The ends justify the means. After all, if you want to make omelettes, you have to break eggs.• Since the English always talk about the weather, if you meet someone who talks about the

weather you can be sure they are from England.• Since many great scientists have believed in God, there must be some truth in religion.• We got on very well on both of our dates together. We are clearly well suited. Let’s get married!• Do you want to be part of the solution or part of the problem?• The average UK family has 2.4 children. The Smiths are very average people. Therefore they must

have 2.4 children.• Since no-one has been able to prove that we are alone in the universe, we must conclude that

other life forms exist.• Are all your family stupid, or is it just you?• No breath of scandal has ever touched the Senator. So he must be an honest man.• Just as you are more likely to take care of a car that you own than one that you rent, so a slave

owner is more likely to take care of his slave than an employer is of his worker.

ToK - Reason

Bad reasoning• Ignorance• Laziness• Pride• Prejudice

ToK - Reason

You are a bar-person in a nightclub. The club has the following rule -

Every person drinking alcohol must be over 20 years of age

There are four situations:

PersonDrinking

Beer

PersonDrinking

Lemonade

PersonAged

19 years

PersonAged

21 years

Which two situations should be investigated?

ToK - Reason

You are presented with the following rule -

Every card with a vowel on one side has an even number on the reverse side

These are the cards:

U G 7 2Which 2 cards should be turned over in order to find out if this rule is indeed the case?

ToK - Reason

Justify your choices

The two examples are formally identical.

Why is it easier to spot the correct answers in the second example?

Do your answers reflect something about human thinking in general?