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Week 7   faulty reasoning - teacher version
 

Week 7 faulty reasoning - teacher version

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    Week 7   faulty reasoning - teacher version Week 7 faulty reasoning - teacher version Presentation Transcript

    • Critical Thinking Faulty Reasoning: FallaciesBernard HoHonBSc, BEd, MScHumber College Institute of Technology and Advanced Learning205 Humber College BoulevardToronto, ON M9W 5L7bernard.ho@humber.ca(416)675-6622 ext. TBA
    • 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 Strict Necessity Test: Is it the arguer’s intention to make the conclusion follow necessarily from the premises? YES NO Deductive Inductive If the premises are If the premises are hypothetically true, do they hypothetically true, do they guarantee the conclusion? make the conclusion probable? YES NO YES NO Valid Invalid Strong WeakAre the premises actually true? Are the premises actually true?YES NO YES NOSound Unsound Cogent Uncogent
    • 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.
    • Fallacies of Relevance: Appeal to the Person• 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.
    • Fallacies of Relevance: Appeal to the Person 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 doctortells 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 RightI 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 acomplete 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.
    • Fallacy of Relevance: Appeal to PopularityMost people agree that owning an SUV is saferthan owning a car. So I guess it must be true.Of course the war is justified. Everyone believedthat it’s justified.The vast majority of Canadians believe thatthere’s a supreme being, so how could you doubtit?
    • Fallacy of Relevance: Appeal to Popularity• 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.
    • Fallacy of Relevance: Appeal to IgnoranceIt’s clear that God exists, because sciencehasn’t proved that he doesn’t exist.You can’t disprove my theory that Bigfoot livesin the forests of B.C. Therefore, my theorystands.
    • Fallacy of Relevance: Appeal to Ignorance• 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 EmotionYou should hire me for this network analystposition. I’m the best person for the job. And if Idon’t get a job soon my wife will leave me, and Iwon’t have enough money to pay for mymother’s heart operation.Officer, there’s no reason to give me a trafficticket for going too fast because I was just on myway to the hospital to see my wife who is in aserious condition to tell her I just lost my job andthe car will be repossessed.
    • Fallacy of Relevance: Scare Tactic• Threatening to harm those who may not accept the arguments 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.
    • Fallacy of Relevance: Red HerringEvery woman should have the right to anabortion on demand. There’s no questionabout it. These anti-abortion activists blockthe entrances to abortion clinics, threatenabortion doctors, and intimidate anyone whowants to terminate a pregnancy.
    • Fallacy of Relevance: Red Herring• 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 militaryspending bill, saying that it’s too costly. Whydoes the NDP always want to slash everythingto the bone? They want a pint-sized militarythat couldn’t fight off a crazed band ofterrorists, let alone a rogue nation!
    • Straw Man?The B.C. Civil Liberties Union has criticized anew anti-porn law because they say itconstitutes unreasonable censorship. As usual,they are defending the porn industry! Theywant to make it easier for sickos to distributekiddy porn. Don’t let them do it. Don’t letthem win yet another battle in defence ofperversion.
    • 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 DilemmaLook, either you support the war, or you are atraitor to your country. You don’t support thewar. 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.
    • 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. – But not all slopes are necessarily slippery. – Some consequences are not inevitable.
    • Fallacy of Insufficient Evidence: Slippery SlopeIf the Federal Government’s “Gun Registry”goes ahead, law-abiding citizens will have toregister their hunting rifles. Next thing youknow, the government will go further andrifles will be banned altogether. And ultimatelyall guns will be banned, and then before long,anything that could be used as a weapon willbe illegal. So if the Gun Registry goes ahead,we might as well turn in our pen-knives andbaseball bats now!
    • Fallacy of Insufficient Evidence: Slippery SlopeOppose increases in tuition! Or next thing you know,tuition will be $20,000 per year!We should not ban child pornography. After all, nextthey’ll ban all pornography, then all erotica, and thenall romance novels!If I give you an extension on your essay just becauseyou had the flu, next thing you know people will wantextensions because they have a hangover!