1、Critical ThinkingLecture 4: Informal fallacies （謬誤）It is easy for us to make mistakes in our reasoning because we are easily affected by psychological emotions, social and political circumstances （including our interests）, and customs and public opinion, and complexity of reasoning itself.One of the

2、 purposes of this course is to learn how to avoid mistakes or fallacies in reasoning.The first thing we need to do is to identify fallacies.1. Fallacies in generalA fallacy is a defect （缺陷） in an argument that consists in something other than merely false premises.A fallacy that involves a mistake i

3、n reasoning is sometimes called anonsequitur （which, in Latin, means“ it does not follow” ）.Two kinds of fallacies: formal and informalIf an argument is unsound or uncogent, then either it has one or more false premises or it commits a fallacy, or both.A formal fallacy is one that may be identified

4、by merely examining the form or structure of an argument. Fallacies of this kind are found only in deductive arguments.Example of Formal FallaciesIf apes are intelligent, then apes can solve the puzzles.Apes can solve the puzzles. Therefore, apes are intelligent.This is an invalid argument.Formal fa

5、llacies are identified purely by analyzing the form.It has the following form:If P, then Q.Q.Therefore, P.Counter example:If it is a rose, then it is a flower.It is a flower.Therefore, it is a rose.Informal fallacies are those that can be detected （檢測到） only by examining the content of the argument.

6、Examples of informal fallaciesA chess player is a person.Therefore, a bad chess player is a bad person.The Brooklyn Bridge is made of atoms.Atoms are invisible. Therefore, the Brooklyn Bridge is invisible.Fallacies of Relevance (意義，關聯)The common feature to all the fallacies of relevance is that the

7、premises are logically irrelevant to the conclusion, though they might be psychologically relevant .1. Appeal to ForceThe arguer tries to get his conclusion accepted by a person or persons by threatening that person or those persons.A girl to her boyfriend:“You should buy me a diamond necoktlhaecerw

8、, ise I willend our relationship.”Secretary to boss: I deserve a raise in salary for the coming year.After all,you know how friendly I am with your wife, and I'm sure you wouldn 't want herto find out what s be'en going on between you and that sexpot client of yours.構成，提出2. Appeal to Pit

9、yInstead of providing the relevant evidence or reason, the arguer tries to evoke sympathy from the audience or the reader to get his conclusion accepted.Taxpayer to judge: Your Honor, I admit that I declared thirteen children as dependents on my tax return, even though I have only two. But if you fi

10、nd me guilty of tax evasion, my reputation will be ruined. I'll probably lose my job, mypoor wife will not be able to have the operation that she desperately needs, and my kids will starve. Surely I am not guilty.3. The Appeal to the People (Emotion)orThe arguer tries to get the conclusion accep

11、ted by playing on the listeners'readers 'desire to be loved, esteemed, admired, valued, or accepted by others or included in a group of people.Play on/upon 利用There are direct and indirect appeal to the people.Let us concentrate on indirect appeal.The bandwagon argument 風靡的活動，時尚If you don 

12、9;t do such and such, so and so, you will be left behind or out of the group.Example:Of course you want to buy Crest toothpaste, because 90 percent of Americans brush with Crest.The appeal to vanity 虛榮If you do such and such, so and so, you will be admired, pursued or imitated. Example:The Few, the

13、Proud, the Marine.The appeal to the snobbery 勢利態度Similar to the appeal to vanity.A Rolls-Royce 勞斯萊斯 is not for everyone. If you qualify as one of the select few, this distinguished classic may be seen and driven at British Motor Cars, Ltd. (By appointment only, please.)Mother to child: You want to g

14、row up and be just like Wonder Woman, don' tyou? Then eat your liver and carrots.4. Argument Against the Person (Argumentum Ad Hominem) The arguer commits this fallacy if in his argument, he directs his attention to another arguer rather than the second arguer ' s argument or position. There

15、 are three kinds of such fallacy: circumstantial, abusive and tu quoqueAd hominem-abusiveThe arguer verbally abuses or attacks another arguer. (offensively or verbal abuse)Poet Allen Ginsberg has argued in favor of abolishing censorship of pornographic literature. But his arguments are nothing but t

16、rash. Ginsberg, you know, is a marijuana-smoking homosexual and a thoroughgoing advocate of the drug culture.Ad hominem-circumstantialThe arguer discredit the opponent ' seanrtgbuymalluding to certain circumstances about the opponent.President George W. Bush argues that we should open up the Arc

17、tic National Wildlife Refuge for oil drilling. But Bush just wants to reward his rich cronies in the oil industry who got him elect ed. Thus, we can hardly take Bush' s argumentseriously.Tu quoque (you too)Child to parent: Your argument that I should stop stealing candy from the corner store is

18、no good. You told me yourself just a week ago that you, too, stole candy when you were a kid.The judge has ruled that we are not paying our employees well enough. How can this be right when the judge' s own secretary doesn' t get paid well either?5. AccidentOne commits Accident if one misapp

19、lies (incorrectly applies) a general principle to a specific case.Dogs have four legs. Fido just had one of his legs amputated. So, Fido is not a dog any more.6. Straw ManThis fallacy is committed when an arguer distorts 歪曲 an opponent '對手，競 爭者 argument for the purpose of more easily attacking i

20、t.Mr. Goldberg has argued against prayer in the public schools. Obviously Mr. Goldberg advocates atheism. But atheism is what they used to have in Russia. Atheism leads to the suppression of all religions and the replacementof God by an omnipotent state. Is that what we want for this country?Ihardly

21、 think so. Clearly, Mr. Goldberg' s argument is nonsense.7. Missing the point (Irrelevant 不相關的 Conclusion)The premises are supposed to lead to one conclusion but a totally different conclusion is drawn.Crimes of theft and robbery have been increasing at an alarming rate lately. The conclusion is

22、 obvious: we must reinstate the death penalty immediately. Certainly Miss Malone will be a capable and efficient manager. She has a great figure, a gorgeous face, and tremendous poise, and she dresses very fashionably.8. Red HerringIt is committed when the arguer diverts the attention of the reader

23、or listener by changing the subject to a different but sometimes subtly related one.The arguer tries to draw the audience or readers off right track and then draw a conclusion on a different issue.).There is a good deal of talk these days about the need to eliminate pesticides from our fruits and ve

24、getables. But many of these foods are essential to our health. Carrots are an excellent source of vitamin A, broccoli is rich in iron, and oranges and grapefruits have lots of vitamin C.We' ve all heard the argument that too much television is the reason our students can 't read and write.Ye

25、t, many of today ' s TV shows are excellent.“ Seinfeld ” explores important issues facing single people,ents“ E.R. ” presmedical professionals in life-and- death situations, and“ 60 minutes ” exposes agreat variety of scams and illegal practices.Today' s TV is just great!偏離軌道Fallacies of Wea

26、k InductionPremises are relevant to the conclusion but not strong enough to draw the conclusion.9. Appeal to Unqualified AuthorityAn argument commits such a fallacy if and only if the arguer appeals to the inappropriate authority that is not the expert in the subject at hand to prove the conclusion

27、instead of relevant evidence or appropriate authority.Pianist Ray Charles says that Sinclair paints are groovy.We can onlyconclude that Sinclair paints are very groovy indeed.Appropriate 正確的恰當的 authority must meet at least two conditions:(1) The expert on the subject.(2) There is agreement among exp

28、erts in that area.“ Why should I be moral? ” must be one of the most important issues in ethics, because professor Stephen Darwall says so, who is one of the leading philosophers in ethics and there are no other moral philosophers哲學家 whodisagree with him.In some areas such as politics, morals, and r

29、eligion, there is no appropriateauthority we can appeal to. For example, Abortion (Homosexuality) is morally wrong because according to the Bible, it is wrong.10. Appeal to Ignorance An argument commits this fallacy if and only if when the premises state that this statement must be true because it h

30、as not been proved false or it is false because it has not been proved true.Arguments are supposed to provide positive evidence.If something isincapable of being proved or disproved, we cannot use that thing to prove or disprove a conclusion.People have been trying for centuries to provide conclusiv

31、e evidence for the claims of astrology, and no one has ever succeeded.Therefore, we mustconclude that astrology is a lot of nonsense.Conversely, People have been trying for centuries to disprove the claims of astrology, and no one has ever succeeded.Therefore, we must conclude that astrology istrue.

32、Two exceptions(1) If qualified researchers investigate a certain phenomenon within their range of expertise and fail to find any evidence, then this search can constitute evidence for a conclusion.Teams of scientists attempted over a number of decades to detect the existence of the luminiferous ethe

33、r, and all failed to do so. Therefore, the luminiferous ether does not exist.(2) If it is an issue of courtroom procedure: Members of the jury, you have heard the prosecution present its case against the defendant. Nothing, however, has been proved beyond a reasonable doubt. Therefore, under the law

34、, the defendant is not guilty.No fallacy since“ Nothing has been proved beyond a reasonable doubt” is what“ not guilty ” means in its legal sense.11. Hasty Generalization (Converse Accident) This fallacy is commited when a general conclusion is drawn from a too small sample (a few cases) or a biased

35、 sample (not representative). On our first date, George had his hands all over me, and I found it nearly impossible to keep him in his place.A week ago Tom gave me that stupid lineabout how, in order to prove my love, I had to spend the night with him.Menare all alike. All any of them wants is sex.1

36、2. False CauseThe connection between the premises and the conclusion depends on an imagined causal connection that does not exist.Three types of false cause: non causa pro causa ( “ not the cause for the cause ” ), “ after this, therefore, because of this ” and oversimplified cause.12a. non causa pr

37、o causa (“not the cause for the cause”)During the past two months, every time that the cheerleaders have worn blue ribbons in their hair, the basketball team has been defeated. Therefore, to prevent defeats in the future, the cheerleaders should get rid of blue ribbons.12b. post hoc ergo propter hoc

38、 (after this, therefore, because of this)A few minutes after Governor Harrison finished his speech on television, a devastating earthquake struck southern Alaska.For the safety of the peopleup there, it is imperative that Governor Harrison make no more speeches.12c. Oversimplified causeThis variety

39、occurs when a multitude of causes is responsible for a certain effect but the arguer selects just one of these causes and represents it as if it were the sole cause.Today, all of us can look forward to a longer life span than our parents and grandparents. Obviously, we owe our thanks to the millions

40、 of dedicated doctors who expend every effort to ensure our health.13. Slippery SlopeIt occurs when the conclusion of an argument rests upon an alleged chain reaction and there is not sufficient reason to think that the chain reaction will actually take place.It is never a good idea to permit teenag

41、ers to drive.Almost at once, they askre infor the family car for a date. They have a few beers, and pretty soon they the back seat having sex. The girl gets pregnant and is forced into the horrendous decision whether to have an abortion, or to live her life as an unmarried mother.Fallacies of Presum

42、ption, Ambiguity,and Grammatical Analogy14. Begging the Question or Circular ReasoningThe fallacy is committed when the arguer creates an illusion that inadequate premises provide adequate support for the conclusion (1) by leaving out a key premise, or (2) by restating the conclusion as a premise or

43、 assuming the truth of the conclusion in the premise(s), or (3) by reasoning in a circle.The latin name for this fallacy, petitio principii, means “ request for the source.(1) Cases of leaving out a key premiseMurder is morally wrong. This being the case, it follows that abortion is morally wrong.Th

44、is argument begs the question“How do you know that abortion is a form ofmurder? ”Clearly, terminally ill patients have a right to doctor assisted suicide.After all,many of these people are unable to commit suicide by themselves.This argument begs 回避 the question “Just because terminally ill patients

45、 cannot commit suicide, why does it follow that they have a right to a doctorassistance?(2) Cases of restating the conclusion as a premiseAnyone who preaches revolution has a vision of the future for the simple reason that if a person has no vision of the future he could not possibly preach revoluti

46、on.Capital punishment is justified for the crimes of murder and kidnapping because it is quite legitimate and appropriate that someone be put to death for having committed such hateful and inhuman acts.(3) Cases for circular reasoningFord Motor Company clearly produces the finest car in the United S

47、tates.Weknow they produce the finest cars because they have the best design engineers. This is true because they can afford to pay them more than other manufacturers. Obviously, they can afford to pay them more because they produce the finest cars in the United States.What is the difference between“

48、 Begging the question ” and valid arguments?The difference depends on whether they create an illusion that inadequate premises provide adequate support for the conclusion or not.15. Complex QuestionYes or no, you have to accept my conclusion.Have you stop beating your spouse yet?Have you stopped che

49、ating on exams?16. False DichotomyThis fallacy is committed when one of the premises is a false dichotomy or a false disjunctive statement.A dichotomy is a pair of alternatives (states, characteristics, or conditions) that are both mutually exclusive and jointly exhaustive.X and Y are mutuallyexclus

50、ive if nothing can be both X and Y.X and Y are jointly exhaustive ifeverything must be either X or Y.Either you buy only Chinese- made products or you don't deserve to be called aloyal Chinese. Yesterday you bought a new Toyota.It 's therefore clear thatyou don 't deserve to be called a loyal Chinese.17. Suppressed EvidenceIf the argument ignores some obvious evidence against it, it commits the fallacy of Suppressed Evidence. For example,Most dogs are