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Critical Reasoning forBeginners: Four

Marianne TalbotDepartment for Continuing EducationUniversity of OxfordMichaelmas 2009

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Last week we learned how toanalyse arguments

and set them out logic-book style

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Six steps to analysing an argument:

1. identify the conclusion;2. identify the premises;3. add suppressed premises4. remove irrelevancies;5. remove inconsistent terms;

6. remove cross-references.

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We saw that, although we need toparaphrase arguments in order tocomplete these steps

we should not change themeaning of any of the premises or

the conclusion

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We also saw that although it is necessaryto bring to bear our understanding of theargument

.it is important not to read into theargument anything that isnt there

..at least implicitly

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It is extremely important, in

analysing an argument, not toevaluate the argument

that comes later.

.first we identify the argument

then we evaluate it.

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This week we shall be starting to

learn how to evaluate arguments

I was going to start with deduction

and so with validity and truth

but I have decided to start with

induction instead

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Inductive arguments are such that.

the truth of their premises

makes the truth of their conclusion

more or less likely

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All inductive arguments rely on theprinciple of the uniformity of nature.

and the only arguments for theprinciple of the uniformity of natureare themselves inductive

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Types of inductive argument:

inductive generalisations;

causal generalisations; arguments from analogy; arguments from authority.

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Inductive generalisations:

The premise identifies acharacteristic of a sample of a

population.

the conclusion extrapolates

that characteristic to the rest of the population.

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60% of the voters

sampled said theywould vote for Mr.Many-Promise.

Therefore Mr. Many-Promise is likely towin.

Whenever I have triedto ring BT it has takenme hours to getthrough.

Therefore when I ringBT today it will takehours to get through.

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Exercise: which questions would you need tohave answered in order to know whether or not

these are good arguments?

60% of the voterssampled said theywould vote for Mr.Many-Promise.

Therefore Mr. Many-

Promise is likely towin.

Whenever I have triedto ring BT it has takenhours.

Therefore today when

I ring BT it will takehours.

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How large is the sample?How many of thosewho would vote inthe election weresampled?

10 out of 1 million?

1000 out of 1million?

How often have I

rung BT in thepast?

Once?About 50 times

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How representative is thesample?

Were the voterssampled all female?Over 40? White?Middle class?Known to the personconducting thesurvey?

Have I only ever rungBT on a Sunday?After 10pm? When Iam in a hurry?

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Are there anycounterexamples?

Has it ever been thecase that 60% of thesample agreedtheyd vote for someone and yetdidnt?

Have I ever rung BTand succeeded ingetting through firsttime?

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Beware informal heuristicsThree of Clubs

Seven of DiamondsNine of DiamondsQueen of HeartsKing of Spades

Ace of Spades

Ace of HeartsAce of ClubsAce of Diamonds

King of Spades

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Beware informal heuristicsIn 4 pages of anovel (2000words) how manywords would youexpect to find

ending in ing?

In four pages of anovel (2000words) how manywords would youexpect to find that

include the letter n?

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Causal generalisations:

The premise identifies acorrelation between two types of event.

the conclusion states thatevents of the first type causeevents of the second type.

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Exercise: which questions would you need tohave answered in order to know whether or not

these are good arguments?

Married men livelonger than singlemen

Therefore beingmarried causes you

to live longer

When air is allowedinto a wound maggotsform

Therefore maggots inwounds are caused

by air being allowedinto the wound

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Is the premise true?

Who says marriedmen live longer?

A married man?A woman who wantsto get married?Fred, whose parents

split up when he was5?

Who says maggotsform when air getsinto a wound?A newly qualifiednurse?An elderly doctor?

A scientific study?

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How strong is the correlation?

How many marriedmen were observed?

Over how long?

Were unmarried men

observed?

How many cases of maggots formingwere observed?

Were wounds intowhich air was not allowed observed?

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Does the causal relation make senseor could it be accidental?

Why would beingmarried cause men tolive longer?

Why would air gettinginto a wound causemaggots to form?

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What causes what?

Could it be that beinglong-lived causesmarriage in men?

Or could having thegenes for longevitycause men to getmarried?

Ercould maggotsforming cause the air toget into a wound?

Or could there besomething that causesboth air getting into thewound and maggots toform?

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Arguments from analogy take justone example of something.

..and extrapolate from a character

of that example.

. to the character of somethingsimilar

to that thing

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The universe is like a pocket-watch

Pocket watches have designers

Therefore the universe must have adesigner

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Evaluating arguments from analogy:

are the two things similar?

are they similar in respect of something relevant?

can we find a disanalogy?

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Arguments from authority

.take one person or group of persons

who are, or are assumed to be, rightabout some things

.and extrapolate to the claim they are

right about other things

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Human rights monitoring organisationsare experts on whether human rightshave been violated.

They say that some prisoners are

mistreated in Mexico.

Therefore some prisoners are mistreated

in Mexico

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Evaluating arguments from authority:

Who exactly is the source of information?

Is this source qualified in the appropriatearea?

Is the source impartial in respect of thisclaim?

Do other experts make other claims?

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Next week well look at validity and truthbefore turning to the evaluation of deductivearguments