Fallacies - PowerPoint Presentation

Slide1

Fallacies

(Really)Slide2

Remember that a fallacy is just an invalid argument. An invalid argument is one where even if the premises are true, the conclusion can still be false. So there are lots of fallacies.

However, there are certain tricky patterns that often fool people. These patterns come to have names.Slide3

Straw Man FallacySlide4

Straw Man Fallacy

The Straw Man Fallacy (sometimes in the UK called “Aunt Sally Fallacy”) is when you misrepresent your opponent, and argue against the misrepresentation, rather than against your opponents claim.Slide5

http://

www.youtube.com/watch?v=xOn7DInBWK4&feature=related

Slide6

Assuming the Original Conclusion

Assuming the original conclusion* involves trying to show that a claim is true by assuming that it is true in the premises. It has the form:

X is true. Why? Because X.

*This is Aristotle’s name for the fallacy.Slide7

Example:

“It

says in the Bible that God exists. Since the Bible is God's word, and God never speaks falsely, then everything in the Bible must be true. So, God must exist

.”Slide8

Example

Premise 1: The bible is God’s word.

Premise 2: God never speaks falsely.

Conclusion: Everything in the bible is true.

Premise 1: Everything in the bible is true.

Premise 2: The bible says that God exists.

Conclusion: God exists.Slide9

Note

The most common name of the fallacy of assuming the original conclusion is “begging the question”. There’s a long story about why that is.

Sometimes people misuse “begging the question” to mean “inviting or raising the question”. You should know that some people look down at you if you do this.Slide10

Mark

Liberman

of Language Log found:

http://languagelog.ldc.upenn.edu/nll/?p=2290

‘if

we search the NYT index for recent uses of "beg the question", we find that out of the first 20 hits, 15 use "beg the question" to mean "raise the question" — and of the five that don't, four are usage articles berating people for misusing the phrase

!’Slide11

False Equivocation

Equivocation (or “false equivocation”) is when one word is used with two meanings in the same argument, rendering it invalid.

Slide12

Silly Example

God is love.

Love is blind.

Ray Charles is blind.

So, Ray Charles is God.Slide13

False Equivocation

If evolution is true, then we should expect that creatures act selfishly.

If evolution is true, then creatures

ought

to act selfishly.

But we know that it’s morally wrong to act selfishly.

Creatures

ought not

to act selfishly.

So evolution is false.Slide14

Begging the Question + Equivocation

“To

allow every man unbounded freedom of speech must always be, on the whole, advantageous to the state; for it is highly conducive to the interests of the community that each individual should enjoy a liberty, perfectly unlimited, of expressing his sentiments” Richard

Whately's

Elements of Logic

(1826)Slide15

Loaded Question Fallacy

Sometimes certain forms of words presuppose certain things. For example:

1. John’s son won the race.

2. John’s son did not win the race.

Both sentences presuppose that John has a son.Slide16

So if I ask you “Did John’s son win the race?” whether you answer ‘yes’ or ‘no’ you are agreeing that John has a son.Slide17

Suppose instead that I ask you:

“At what age did you first use drugs?”

Any answer to this question is an admission that you used drugs at some point. Slide18

Or consider:

“Have you stopped beating your wife?”

‘I stopped beating my wife’ and ‘I didn’t stop beating my wife’ both presuppose that at some point in the past, you beat your wife. You can’t answer this question without admitting guilt.Slide19

Masked Man FallacySlide20

Inderscernability of

Identicals

Normally, if you have two things X and Y, but X and Y are really one thing, because X = Y, then if something is true of X, it’s true of Y. For example:

Confucius was the greatest Chinese philosopher.

Confucius =

Kongzi

.__________________

Therefore,

Kongzi

was the greatest Chinese philosopher.Slide21

Indiscernibility of Identicals

This also means that if something is true of X, and it’s not true of Y, then X and Y are different things:

This gas is deadly to humans.

Oxygen is not deadly to humans.

Therefore this gas ≠ not oxygen.Slide22

Masked Man Fallacy

Sometimes, however, this argument doesn’t work:

I know who Bruce Wayne is.

I don’t know who Batman is.

Therefore, Bruce Wayne ≠ Batman.Slide23

More serious example…

I know that I have a mind.

I don’t know that I have a

brain

._____________

Therefore, the mind ≠ brain.Slide24

False Dilemma

An argument commits the false dilemma fallacy when it presents two options as the only options, even though there are actually more options.Slide25

False Dilemma

Premise 1: We can either raise taxes on everyone, or cut social programs.

Premise 2: Raising taxes on the poor would be terrible, they can’t afford it.

Conclusion: We should cut social programs.Slide26

Fallacy of the Mean

The fallacy of the mean is the assumption that a “middle point” between two views is the right one.Slide27

Fallacy of the Mean

Candidate 1: “We should raise taxes on everyone”

Candidate 2: “We should cut social programs”

Therefore,

Compromise: We should raise taxes on everyone a little and cut social programs a little.Slide28

Distribution Fallacy

The distribution fallacy is committed when one assumes that individuals have the properties of groups to which they belong.

Lingnan

has an excellent philosophy department.

I am a philosopher at

Lingnan

._________

Therefore, I am an excellent philosopher.Slide29

Distribution Fallacy

Kooks and quacks will often try to make their theories sound better “by association”:

Having a PhD.

Making one’s work sound “science-y”.

Debating serious scholars.

Associating oneself with respectable institutions (Stanford, Smithsonian, etc.)Slide30

Composition Fallacy

The converse of the distribution fallacy is the composition fallacy, assuming that groups have the properties of the individuals that compose them.

For example: “A point doesn’t have any length; lines are made out of points; therefore, a line doesn’t have any length.”Slide31

Condorcet Paradox

One example of the composition fallacy is the Condorcet Paradox, where every voter can have rational preferences (doesn’t prefer A to B, B to C, and prefer C to A), but the preferences of all the voters taken together are irrational.Slide32

Condorcet Paradox

First Choice

Second Choice

Third Choice

Voter #1

George

Bill

Barry

Voter #2

Barry

George

Bill

Voter #3

Bill

Barry

GeorgeSlide33

Condorcet Paradox

Here, the preferences of the group are irrational:

A majority like George better than Bill.

A majority like Bill better than Barry.

A majority like Barry better than George.Slide34

Ecological Fallacy

Here’s an “ecological inference”.

Countries where, on average, people consume more fat have higher rates of breast cancer.

Therefore, consuming more fat leads to a higher risk of breast cancer.Slide35

There’s a potential problem here with “confounding variables”.

Maybe countries that consume more fat, on average, are also countries that have more pollution, on average (perhaps because pollution and fat consumption both correlate with poverty). So maybe it’s the pollution and not the fat that causes breast cancer.Slide36

Ecological Fallacy

But let’s assume we know there aren’t any confounding variables. Does the premise support the conclusion:

Premise: Countries that on average consume more fat on average have higher rates of breast cancer.

Conclusion: Consuming fat leads to a higher risk for breast cancer.Slide37

Ecological Fallacy

But the conclusion doesn’t follow.

Suppose that in Country A (10 people):

5 people eat 4 pounds of fat a day.

5 people eat 0 pounds of fat a day.

Average fat consumption: 2 pounds/ day.Slide38

Ecological Fallacy

In Country B (also 10 people):

5 people eat 2 pounds of fat/ day

5 people eat 1 pound of fat/ day

Average fat consumption 1.5 pounds fat/ day.

Country B on average consumes less fat.Slide39

Ecological Fallacy

Now assume that in Country A, all 5 people who consume

no

fat get breast cancer. And in

C

ountry B, no one gets breast cancer.

So on average, Country B consumes less fat and has a lower rate of breast cancer. Country B consumes more fat and has a higher rate of breast cancer. Slide40

Ecological Fallacy

But still, this doesn’t mean people who consume more fat are more likely to get breast cancer.

It’s the people who consume

no

fat that get cancer!Slide41

Ecological Fallacy

A famous (purported) instance of the ecological fallacy was Durkheim’s argument that since suicide rates in Catholic countries were lower than in Protestant countries, Catholics were less likely to commit suicide than Protestants.Slide42

Prosecutor’s Fallacy

Suppose you are arrested on the basis of some evidence– you have very large feet, just like the footprints we found at the scene of the crime.

If someone is the killer, there’s a 100% chance that they have very large feet.

If someone is not the killer, there’s a 95% chance they don’t have very large feet.Slide43

Prosecutor’s Fallacy

The Prosecutor’s Fallacy is to assume that therefore you must be guilty.

Why does this not follow?

What other fallacy (already discussed) is identical to the Prosecutor’s Fallacy?Slide44

Argument from Ignorance

The argument from ignorance goes like this:

“You can’t prove that God doesn’t exist. Therefore God exists.”

It assumes that because there is no argument

against

a position, that that position must be correct.Slide45

Shifting the Burden of Proof

A similar fallacy is “shifting the burden of proof”. It goes:

“God exists. If you think otherwise,

prove

that he doesn’t!”

Here, you make a claim (“God exists”) but instead of giving evidence for it, you require that your opponent give evidence for the opposite.Slide46

Genetic Fallacy

The genetic fallacy seeks to evaluate a claim on the basis of its origin.

So, for example, someone might say, “Eugenics is wrong, because the Nazis began it and did horrible things for its sake.”

Eugenics may be wrong, but the fact that the Nazis began it is irrelevant to

this claim.Slide47

Genetic Fallacy

The genetic fallacy seeks to evaluate a claim on the basis of its origin.

So, for example, someone might say “Clearly God does not exist. The reason I know this is that your argument for his existence is fallacious. Since you provided a fallacious argument that God exists, it follows that God does not exist.”Slide48

Appeal to Motive

Sometimes people argue that a certain claim must be false, or an argument invalid, because of the motives of the person making the claim/ argument.Slide49

Appeal to Motive

For example:

“My opponent claims that the government should give free cookies to everyone. But he stands to benefit most, because he likes cookies so much!”Slide50

Tu

Quoque

“

Tu

quoque

” is Latin for “you too”. It’s a defense of an invalid argument that goes:

“You’ve made a similar argument. So you cannot criticize the flaws in this argument.”

Just because other people are doing it doesn’t make

it right!