Week 7 faulty reasoning - teacher version

Critical Thinking

Faulty Reasoning: Fallacies

Bernard HoHonBSc, BEd, MScHumber College Institute of Technology and Advanced Learning205 Humber College BoulevardToronto, ON M9W [email protected](416)675-6622 ext. TBA

mailto:[email protected]

Outline

• Fallacies in general– Weaknesses in arguments

• Fallacies of relevance– Arguments with irrelevant premises

• Fallacies of insufficient evidence– Arguments with unacceptable premises

Arguments Try to Prove a Point

• If an argument is good, it is good no matter who makes it.

• Arguments are good or bad because of their own intrinsic strengths or weaknesses, not because of who offers them up.

• An argument can fail because:– The reasoning is faulty (invalid or weak);– The premises are false (unsound or uncogent);– Or both.

Arguments

InductiveDeductive

Valid Invalid Weak

Sound Unsound Cogent Uncogent

Strong

Strict Necessity Test: Is it the arguer’s intention to make the conclusion

follow necessarily from the premises?

YES NO

If the premises are hypothetically true, do they guarantee the conclusion?

YES NO

If the premises are hypothetically true, do they

make the conclusion probable?

Are the premises actually true?

YES NO

YES NO YES NOAre the premises actually true?

Fallacies

• Fallacies can seem plausible and persuasive, but really make no logical sense.

• You need to study fallacies to:– Avoid committing them;– Detecting when others do it.

Two Categories of Fallacies

• Fallacies of relevance– Arguments that use premises that have nothing to

do with the conclusion.– Premises are irrelevant.

• Fallacies of insufficient evidence– Arguments with premises that are relevant to the

conclusion but still dubious.– Premises fail to provide enough support for the

conclusion.

Fallacies of Relevance

• These fallacies have premises that are irrelevant to the conclusion.– Genetic fallacy– Appeal to the person (Personal attack)– Attacking the motive– Tu quoique (Look who’s talking)– Two wrongs make a right– Appeal to popularity (Bandwagon)– Appeal to ignorance– Appeal to emotion (Appeal to pity)– Red herring– Straw man

Fallacies of Insufficient Evidence

• These fallacies have unacceptable premises:– Begging the question– False dilemma– Slippery slope– Hasty generalization– Faulty analogy– Questionable cause

Fallacy of Relevance: Genetic Fallacy

• Arguing that a claim is true or false solely because of its origin.

Taylor’s argument regarding the existence of God can’t be right because she’s an atheist.

We should reject that proposal for solving the current welfare mess. It comes straight from the Conservative Party.

Russell’s idea about tax hikes came to him in a dream, so it must be a stupid idea.

Fallacy of Relevance: Genetic Fallacy

• These arguments fail because they reject a claim based solely on where it comes from, not on its merits.

• Judging a claim only by its source is a recipe for error. Think of it this way:– A good argument presented by a moron is still a

good argument.– A bad argument presented by a genius is still a

bad argument.

Fallacy of Relevance: Appeal to the Person

• Rejecting a claim by criticizing or discrediting the person who makes it rather than the claim itself.

• Also called an ad hominem or personal attack.

X is a bad or disreputable person.Therefore, X’s argument must be faulty.

Fallacies of Relevance: Appeal to the Person

• Common in criminal court (watch LAW & ORDER)• Prosecutors and defence attorneys often try to

weaken their opponent’s case by discrediting their witnesses.

Dr. Raza testified that Dr. Auster’s alcoholism led Auster to incorrectly prescribe medication for Suzanne Morton, thus causing her death. But Dr. Raza earned his medical degree from the University of Peshawar and has only practiced medicine for a few years. His argument, therefore, is worthless.


• These arguments fail because they attempt to discredit a claim by appealing to something that is almost always irrelevant to it: a person’s character, motives, or personal circumstances.

• They say nothing about the quality of the argument.


Dr. Raza testified that Dr. Auster’s alcoholism led Auster to incorrectly prescribe medication for Suzanne Morton, thus causing her death. But Dr. Raza is only testifying so that he avoids being charged with falsifying medical documents. Therefore, his argument should be rejected.

• Sometimes, it is reasonable to doubt a person’s premises because of who they are.– When you have reason to expect bias.– When they seem to lack relevant expertise.

Fallacy of Relevance: Tu Quoique

• Also called Look Who’s Talking• Rejecting a person’s argument or claim

because that person fails to “practice what they preach”

X fails to follow his/her own advice.Therefore, don’t believe his advice.

Tu Quoique?

I won’t stop smoking just because my doctor tells me to. He won’t stop smoking either!– This is a fallacy.

I should stop smoking like my doctor told me; but so should my doctor! – This is not a fallacy because no argument is

being rejected on the basis of the arguer being a hypocrite.

Fallacy of Relevance: Two Wrongs Make a Right

• Trying to make a wrong action look right, by comparing it to another wrong (perhaps worse) action.

X is as bad or worse than Y. Therefore Y is not wrong.

Fallacy of Relevance: Two Wrongs Make a Right

I don’t feel guilty about cheating on Dr. Boyer’s test. Half the class cheats on his tests.

Why pick on me, Officer? Nobody comes to a complete stop at that sign.

Two Wrongs Make a Right?

• Sometimes actions can be justified by the fact that other actions have taken place.

I killed the man because he was about to kill me. It was an act of self-defense.

I jumped into the pool when it was closed and off-limits because my friend jumped in and was drowning.

Fallacy of Relevance: Appeal to Popularity

• Arguing that a claim must be true merely because a substantial number of people believe it.

• Also called bandwagon argument.

Everyone (or almost everyone, most people, many people) believes X.So X is true.


Most people agree that owning an SUV is safer than owning a car. So I guess it must be true.

Of course the war is justified. Everyone believed that it’s justified.

The vast majority of Canadians believe that there’s a supreme being, so how could you doubt it?


• These arguments fail because they assume that a proposition is true merely because a great number of people believe it.

• But as far as the truth of a claim is concerned, what many people believe is irrelevant.

Appeal to Popularity?

Of course smoking causes cancer! Everybody says so!

• A fallacy is a mistake of reasoning.• An argument can use faulty reasoning, but it

can still have a true conclusion.

Appeal to Popularity?

• Not all appeals to popular beliefs or practices are fallacious.

• If the premises are relevant to the conclusion, these arguments are not fallacious.

All the villagers say that the water is safe to drink. Therefore, the water is probably safe to drink.

Fallacy of Relevance: Appeal to Ignorance

• Arguing that a lack of evidence proves something.

• The problem arises by thinking that a claim must be true because it hasn’t been shown to be false.

No one has shown that ghosts aren’t real, so they must be real.


It’s clear that God exists, because science hasn’t proved that he doesn’t exist.

You can’t disprove my theory that Bigfoot lives in the forests of B.C. Therefore, my theory stands.


• Lack of evidence alone can neither prove nor disprove a proposition.

• Lack of evidence simply reveals our ignorance about something.

Burden of Proof

• Appeals to ignorance involve the notion of burden of proof. – Burden of proof is the weight of evidence of

argument required by one side in a debate or disagreement.

– Problems arise when the burden of proof is placed on the wrong side.

Burden of Proof

• Usually rests on the side that makes a positive claim.

• If you think that psychics exist, you bear the burden of proof.

• If you think X causes cancer, you usually bear the burden of proof.

Fallacy of Relevance: Appeal to Emotion

• Using emotions as premises in an argument.• Trying to persuade someone of a conclusion

primarily by arousing his or her feelings, rather than presenting relevant reasons.

• These arguments fail because emotions are irrelevant to the conclusion.

Fallacy of Relevance: Appeal to Emotion

You should hire me for this network analyst position. I’m the best person for the job. And if I don’t get a job soon my wife will leave me, and I won’t have enough money to pay for my mother’s heart operation.

Officer, there’s no reason to give me a traffic ticket for going too fast because I was just on my way to the hospital to see my wife who is in a serious condition to tell her I just lost my job and the car will be repossessed.

Fallacy of Relevance: Scare Tactic

• Threatening to harm those who may not accept the argument's conclusion.

• The threat is irrelevant to the conclusion.

If you don’t accept what I say something bad will happen. Therefore, what I say is true.

This gun control bill is wrong for America, and any politician who supports it will discover how wrong s/he is at the next election

Scare Tactic?

• Not all threats are fallacies.• If the threat is a natural consequence of an act

or belief, then the threat is relevant.

You should not pass that law because it will hurt the public welfare.

Fallacy of Relevance: Red Herring

• Raising an irrelevant issue and then claiming that the original issue has effectively been settled.

• The irrelevant issue is just a distraction. • All fallacies of relevance are red herrings, but

reserve this to describe fallacies that do not fit into the other categories.


Every woman should have the right to an abortion on demand. There’s no question about it. These anti-abortion activists block the entrances to abortion clinics, threaten abortion doctors, and intimidate anyone who wants to terminate a pregnancy.


• The last part of the argument may be true, and it may be bad, but it’s not relevant.

• The issue is whether women should have the right to abortion on demand.

• The arguer shifts the subject to the behaviour of anti-abortion activists, as though their behaviour has some bearing on the original issue.

• Their behaviour, of course, has nothing to do with the main issue.

Fallacy of Relevance: Straw Man

• Distorting, weakening, or oversimplifying someone’s position so that it can be more easily attacked or refuted.

Distort a claim. Refute the distorted claim.

Straw Man?

The Opposition is opposed to the new military spending bill, saying that it’s too costly. Why does the NDP always want to slash everything to the bone? They want a pint-sized military that couldn’t fight off a crazed band of terrorists, let alone a rogue nation!

Straw Man?

The B.C. Civil Liberties Union has criticized a new anti-porn law because they say it constitutes unreasonable censorship. As usual, they are defending the porn industry! They want to make it easier for sickos to distribute kiddy porn. Don’t let them do it. Don’t let them win yet another battle in defence of perversion.

Fallacies of Insufficient Evidence

• These fallacies have unacceptable premises:– Begging the question– False dilemma– Slippery slope– Hasty generalization– Faulty analogy– Questionable cause

Fallacy of Insufficient Evidence: False Dilemma

• Asserting that there are only two alternatives to consider in some issue when there are actually more than two.

Fallacy of Insufficient Evidence: False Dilemma

Look, either you support the war, or you are a traitor to your country. You don’t support the war. So you’re a traitor.

– This argument only works if there really are only two alternatives.

– Because this argument does not allow other possibilities, it is fallacious.

– If you can think up more possibilities, then you may be looking at a false dilemma.

Fallacy of Insufficient Evidence:Slippery Slope

• Arguing, without good reasons, that taking a particular step will inevitably lead to a further, undesirable step (or steps).

• If you take the first step on a slippery slope, you will have to take others because the slope is slippery.


• Arguing, without good reasons, that taking a particular step will inevitably lead to a further, undesirable step (or steps).

• If you take the first step on a slippery slope, you will have to take others because the slope is slippery. – But not all slopes are necessarily slippery.– Some consequences are not inevitable.


If the Federal Government’s “Gun Registry” goes ahead, law-abiding citizens will have to register their hunting rifles. Next thing you know, the government will go further and rifles will be banned altogether. And ultimately all guns will be banned, and then before long, anything that could be used as a weapon will be illegal. So if the Gun Registry goes ahead, we might as well turn in our pen-knives and baseball bats now!


Oppose increases in tuition! Or next thing you know, tuition will be $20,000 per year!

We should not ban child pornography. After all, next they’ll ban all pornography, then all erotica, and then all romance novels!

If I give you an extension on your essay just because you had the flu, next thing you know people will want extensions because they have a hangover!

Spiritual

Week 7 faulty reasoning - teacher version