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WAYS OF

KNOWING

SOURCE OF KNOWLEDGEOr

OBSTACLE TO IT?

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REASON This is a way of

acquiring new knowledge about the world

We go beyond the immediate evidence of the senses

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So, how good is your reasoning?

A man rides into town on Friday, he stays three night and leaves on Friday. HOW?HOW?

Antony and Cleopatra are lying dead on the floor of a villa in Egypt. Nearby there is a broken bowl. There is no mark on either of them and they were not poisoned. HOW DID THEY DIE?HOW DID THEY DIE?

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And the answer is……meet

Antony &

Cleopatra

And Friday

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Finally Rationalism and certainty? All human beings are mortal

Socrates is a human being It necessarily follows that

Socrates is mortal No if’s’ or but’s’, no opinions,

nothing to do with culture Given the assumptions

or premise the conclusion has to follow

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What is rationalism? The central tenet is that we can

discover important truths through reason alone

Rationalist like

Logic ] certain and useful, Maths ] unlike empiricism

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Three kinds of reasoning There are three kinds

of reasoning Deductive Inductive Informal

All have serious fallacies – invalid patterns of reasoning that need to be considered and guarded against

So, get your brains into gear and start thinking

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Deductive reasoning

general - specific

All apples are fruit Some apples are red Therefore Some fruit is red

This is a syllogism Syllogisms have1. 2 premises and a

conclusion2. 3 terms that each

appear twice3. QUANTIFIERS – all,

some, no – the quantity that is being referred to

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Truth and validity – crucial diff

These are NOT the same thing Truth = what is the case, it is

property of the statement, validity of arguments

Validity = whether the conclusion follows the premises

So an argument is valid or invalid NOT true or false

STAY WITH ME ON THIS ONE

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The validity of an ARGUMENT is INDEPENDENT of the truth or falsity of the premises

+ =

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You can have false premises and true conclusions

WHAT DO YOU RECKON THIS ONE IS?WHAT DO YOU RECKON THIS ONE IS?

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Mission Impossible is – valid argument with true premises and

false conclusion In groups of 4 peopleIn groups of 4 people Construct syllogisms that haveConstruct syllogisms that have

2 true premises, and a true conclusion2 true premises, and a true conclusion 1 true premise, 1 false premise and a true conclusion1 true premise, 1 false premise and a true conclusion 1 true premise, 1 false premise and a false 1 true premise, 1 false premise and a false

conclusionconclusion 2 false premises, and a true conclusion2 false premises, and a true conclusion 2 false premises, and a false conclusion2 false premises, and a false conclusion

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Argument structure Pure logic is concerned with the structure of the

arguments not the content All dogs are mammals Some dogs are Boxers THEREFORE – some mammals are Boxers

This reduces to All A’s are B Some A’s are C Therefore some C are B’s

This abstraction helps avoid belief bias – belief that an argument is valid because we

agree with the conclusion

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Venn diagrams, should help with this

Syllogisms are no easy matter to sort – Venn diagrams can be used to help (Maybe)

You build up the Venn diagram from the premises then you can work out if the conclusion is correct

Now you tryNow you try All A’s are B’s All A’s are B’s All B’s are C’s All B’s are C’s Therefore Therefore

all C’s are A’sall C’s are A’s

BEWARE

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Over to you Over to you again again Use Venn diagrams to state whether each of Use Venn diagrams to state whether each of

these are valid or invalidthese are valid or invalid All Italians eat spaghettiAll Italians eat spaghetti Giovanni Rossi eats spaghettiGiovanni Rossi eats spaghetti Therefore Giovanni Rossi is ItalianTherefore Giovanni Rossi is Italian

No Martians have red nosesNo Martians have red noses Rudolph has a red noseRudolph has a red nose Therefore Rudolph is not a MartianTherefore Rudolph is not a Martian

Some monks are Tibetan Some monks are Tibetan All Tibetans are good at YogaAll Tibetans are good at Yoga Therefore some monks are good at yogaTherefore some monks are good at yoga

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And in conclusion (just in case your not totally confused)

Just because the argument is valid – it does not mean the conclusion is true

For the conclusion to be true you must be able to answer yes to both of these Are the premises true? Is the argument valid?

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The premise is obvious = assume the rest In everyday life we rarely argue formally

and some things are missing BUT WHATBUT WHAT

Jenny goes to Oxford University, so she must Jenny goes to Oxford University, so she must be very intelligentbe very intelligent

Graham is a politician so he is probably lyingGraham is a politician so he is probably lying Since it is natural to eat meat there is nothing Since it is natural to eat meat there is nothing

morally wrong with itmorally wrong with it

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However!! Deductive reasoning preserves the

truth – it is NOT a source of truth All human beings are mortal

Socrates is a human being Therefore Socrates is mortal This is true if the premises are true –

knowledge of mortality is not from reasoning it is based on experience

AND SO TO

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Inductive reasoningspecific to

general

How do we know humans are mortal?

All humans in history have died therefore through inductive inference we move form the observed to the unobserved

All observed humans are mortal so ALL humans are mortal

We use this all the time in everyday life

Past experience shapes our expectations

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Language is based on inductive reasoning HOW?HOW?

My cat gets excited when I go to the fridge

is he using is he using

inductive reasoning?inductive reasoning? Do animals reason?Do animals reason?

Or is it something else?Or is it something else?

SCIENCE USES INDUCTIVE REASONING It formulates general laws from observations

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Deduction vs. Induction General – particular

Example Water is a liquid Liquids turn to a gas

when heated Water will turn to a

gas if heatedValue

More certain, less informative

Particular to generalExample

Liquid A turns to a gas when heated, liquid B turns to a

gas when heated etc All liquids turn to

gas when heatedValue

More informative but less certain

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How reliable is inductive reasoning?

The problem here is hasty generalisation Sexism Racism

What is the boiling point of water?

Hasty generalisations are made worse by confirmation bias

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What hasty generalisations do you What hasty generalisations do you make?make?

Are prejudice, generalisations and Are prejudice, generalisations and scientific law different?scientific law different?

Read this carefullyRead this carefullyA businessman has just turned off the A businessman has just turned off the

lights in the store when a man lights in the store when a man appeared and demanded money. The appeared and demanded money. The owner opened a cash register . The owner opened a cash register . The contents of the cash register were contents of the cash register were

scooped up and the man sped away. A scooped up and the man sped away. A member of the police force was notified member of the police force was notified

promptly. promptly.

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Good or bad generalisation

? Certain criteria make

for more reliable generalisations Number – see 1 dog

swimming it is not enough to conclude that all can swim

Variety – various circumstances, different breeds of dog, young, old

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Exceptions – avoid confirmation bias and actively look for counter-examples, ask friends for them?

Coherence – more evidence is needed to support strange claims

Subject area – natural sciences are more likely to yield reliable generalisations than human sciences

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