probablistic arguments ch.3 1.2012

8/2/2019 Probablistic Arguments Ch.3 1.2012

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Tools for Critical Thinking:The Structure of Probabilistic

Arguments

*Adapted from Elliot Sobers Core Questions in Philosophy (Prentice Hall: 2009)


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If Deductively validarguments have perfect

strengthWhy bother with any other argument form?


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Limitations of Deductively ValidArguments

1) The Duh factor. Many times DV arguments tell ussomething quite obvious.

2) Their scope limits us. While this form includes anabsolute guarantee of truth, many importantquestions do not have any guaranteed answers.


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Types of Good Arguments


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Form #2: Probabilistic Arguments

When we use induction and abduction, we aretaking a bit of a gamble, because we arerelying on probabilities.

Probabilistic argument forms enable us tomake predictions and generalizations.

Probabilistic Arguments are never perfectly

strong, but they can be very, very strong.


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There is a whole range of strengths an

probabilistic argument can have Remember, deductive

validity (perfect strength)is an on/off concept.

By contrast, probabilisticstrength comes in manylevels. Just as someonecan be really loud, sort ofloud, or not at all loud, anprobabilistic argument can

be really strong, sort ofstrong, or not at all strong.


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Inductive Argument #1Premise: The sun rose

this morning.

Premise: The sun roseyesterday morning.

Premise: The sun rose

two mornings ago.

Premise: The sun rosethree mornings ago.Conclusion: The sun will rise tomorrow.

(A prediction.)


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The odds are incredibly high that our prediction will come true, but there is

a chance that the conclusion will turn out to be false.

But predicting that the sun will rise

tomorrow doesn

t seem like agamble


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Despite the risk, we need to make some conclusion about whether the sun

will rise tomorrow, which is why we use inductive argument forms.

Question: What other possibilities are there besides

the sun rising tomorrow (no matter how improbable)?


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Example #2: We use inductive

arguments constantlyPremise: I touchedthe hot stoveyesterday and it

burnt my hand.Premise: I touched

the hot stove twodays ago, and it

burnt my hand.

Conclusion: The stove will

always burn my hand.


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We lack guarantees even about

matters we never question Suppose that you know a

woman who resemblesyou and has taken care of

your entire childhood. Shemay tell you that she isyour biological Mom.

Is there any way to becertain that the woman

who says she is yourbiological Mom truly isyour biological Mom?Why or why not?

Im yourMom!


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Example #3: A final example of aninductive argument

Premise: This Coca-Cola beverage is caffeine-free.

Conclusion: All Coca-Cola beverages are caffeine-free. (A generalization.)

Question: Do you think there is a strong or a weak relationship between

the premise and the conclusion?


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Deductive and ProbabilisticReasoning

When we call an argument inductively/abductively strongor deductively valid, this describes its structure and not itscontent.

All deductive arguments have perfect strength, but not alldeductive arguments are good.

Inductive arguments always have less than perfectstrength, but they can be very good arguments.

Even if you have never realized it before, these twomethods for building an argument are the building blockswe use whenever we think rationally.


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Recognizing an InductiveArgument

The conclusion is NOT guaranteed to be true even if allthe evidence offered is true.

The conclusion answers a what question

Its conclusion is a generalization or a prediction


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Evaluating an InductiveArgument

Is the sample given in the premises big enough?

Are the examples REPRESENTATIVE?


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Recognizing an AbductiveArgument

The conclusion is NOT guaranteed to be true even if allthe evidence offered is true.

The conclusion answers a why question

The conclusion gives an explanation of the evidence

Detectives, Auto Mechanics, Physicians

probablistic arguments ch.3 1.2012

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