Logical Fallacies逻辑谬误.ppt

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1、Logic(part four)&FallaciesArguments,like men,are often pretenders.PlatoWhat are fallacies?What are fallacies?&Fallacies are defects that weaken arguments.&By learning to look for them in your own and others writing,you can strengthen your ability to evaluate the arguments you make,read,and hear.two

2、special notes about fallaciestwo special notes about fallacies&Fallacious arguments are very,very common and can be quite persuasive,at least to the casual reader or listener.You can find dozens of examples of fallacious reasoning in newspapers,advertisements,and other sources.&There are many differ

3、ent kinds of fallacies with different names.&Argument from IgnoranceDefinitionDefinition&Argument from Ignorance asserts that a proposition is true because it has not been proved false,or that it is false because it has not been proved true.&In science and logic,we must verify the truth or falsehood

4、 of propositions;before that,nothing can be asserted.&In jurisprudence,however,there is an exception:a person is always regarded as innocent before he/she is proved guilty.ExamplesExamples&You cant prove that God exists.Therefore he doesnt exist.&You cant prove that God doesnt exist.Therefore he exi

5、sts.&the story of Galileo&Hasty GeneralizationDefinitionDefinition&making assumptions about a whole group or range of cases based on a sample that is inadequate(usually because it is atypical or just too small).Stereotypes about people(“pilot students are handsome,grad students are nerdy,etc.)are co

6、mmon examples of hasty generalization.ExampleExample&My roommate said her philosophy class was hard,and the one Im in is hard,too.All philosophy classes must be hard!Two persons experiences are,in this case,not enough on which to base a conclusion.&Smoking causes cancer because my father was a smoke

7、r and he died of lung cancer.TipTip&Ask yourself what kind of sample youre using:Are you relying on the opinions or experiences of just a few people,or your own experience in just a few situations?If so,consider whether you need more evidence,or perhaps a less sweeping conclusion.&Notice that in the

8、 previous example,the more modest conclusion Some philosophy classes are hard for some students would not be a hasty generalization.&Accidentdefinitiondefinition&When we apply a generalization to individual cases that it does not properly govern,we commit the fallacy of accident.Almost every good ru

9、le has appropriate exceptions;we argue fallaciously when we reason on the supposition that some rules apply with universal force.Accident is the opposite of false generalization.ExampleExample&I am sure their ambassador will be reasonable about the matter.After all,man is a rational animal.Man is a

10、rational animal.Thats true,because we have reason.But that does not mean all men behave rationally under all circumstances.“All generalizations are false,including this one.”Mark Twain&False cause/Post hoc(假性因果假性因果)DefinitionDefinition&assuming that because B comes after A,A caused B&Of course,somet

11、imes one event really does cause another one that comes later.for example,if I register for a class,and my name later appears on the roll,its true that the first event caused the one that came later.&But sometimes two events that seem related in time arent really related as cause and effect.That is,

12、correlation isnt the same thing as causation.ExamplesExamples&President Jones raised taxes,and then the rate of violent crime went up.Jones is responsible for the rise in crime.The increase in taxes might or might not be one factor in the rising crime rates,but the argument hasnt shown us that one c

13、aused the other.&A flood happened after the comet appeared;therefore,the comet caused the flood.&misunderstanding of the lunar eclipseTipsTips&To avoid the post hoc fallacy,the arguer would need to give us some explanation of the process by which the tax increase is supposed to have produced higher

14、crime rates.&Thats what you should do to avoid committing this fallacy:If you say that A causes B,you should have something more to say about how A caused B than just that A comes first and B comes later!&Weak/False Analogy(错误类比错误类比)DefinitionDefinition&Many arguments rely on an analogy between two

15、or more objects,ideas,or situations.If the things that are being compared arent really alike in the relevant respects,the analogy is a weak one,and the argument that relies on it commits the fallacy of weak analogy.ExampleExample&Guns are like hammerstheyre both tools with metal parts that could be

16、used to kill someone.And yet it would be ridiculous to restrict the purchase of hammersso restrictions on purchasing guns are equally ridiculous.While guns and hammers do share certain features,these features(having metal parts,being tools,and being potentially useful for violence)are not the ones a

17、t stake in deciding whether to restrict guns.Rather,we restrict guns because they can easily be used to kill large numbers of people at a distance.This is a feature hammers do not shareitd be hard to kill a crowd with a hammer.Thus,the analogy is weak,and so is the argument based on it.ExamplesExamp

18、les&Mind and rivers,can be both broad.It is a known fact that the broader the river,the shallower it is.Therefore it must be true,that the broader the mind is,the shallower it is.&Employees and nails are the same.As it is necessary to hit nails on the head in order to make them work,the same must be

19、 done with employees.TipTip&Identify what properties/attributes are important to the claim youre making,and see whether the two things youre comparing both share those properties.&This shows the importance of relevance.&Appeal to inappropriate authority(诉诸诉诸不当不当权威权威)DefinitionDefinition&Often we add

20、 strength to our arguments by refer-ring to respected sources or authorities and explaining their positions on the issues were discussing.This is quite proper and even encouraged in academic research.&If,however,we try to get readers to agree with us simply by impressing them with a famous name or b

21、y appealing to an authority who has no special competence in the sphere concerned,we commit the fallacy of appeal to inappropriate authority.ExampleExample&We should abolish the death penalty.Many respected people,such as actor Mr.Handsome,have publicly stated their opposition to it.&The most blatan

22、t examples of misplaced appeals to authority appear in advertising“testimonials.”TipsTips&There are two easy ways to avoid committing appeal to inappropriate authority:first,make sure that the authorities you cite are experts on the subject youre discussing;second,rather than just say Dr.Authority b

23、elieves x,so we should believe it,too,try to explain the reasoning or evidence that the authority used to arrive at his or her opinion.&It also helps to choose authorities who are perceived as fairly neutral or reasonable,rather than people who will be perceived as biased.&Ad populum/Appeal to Peopl

24、e(诉诸诉诸群众群众)DefinitionDefinition&The arguer takes advantage of the desire most people have to be liked and to fit in with others and uses that desire to try to get the audience to accept his or her argument.One of the most common versions is the bandwagon fallacy(主流思想谬误),in which the arguer tries to

25、convince the audience to do or believe something because everyone else(supposedly)does.&It is also very commonly used in advertisements.ExampleExample&Gay marriages are just immoral.70%of Americans think so.&Why are so many people attracted by the Pontiac Grand Prix?It could be that so many people a

26、re attracted by the Grand Prix becauseso many people are attracted by the Grand Prix!TipTip&Make sure that you arent recommending that your audience believe your conclusion because everyone else believes it,all the cool people believe it,people will like you better if you believe it,and so forth.Kee

27、p in mind that the popular opinion is not always the right one!“The fact that an opinion has been widely held is no evidence whatsoever that it is not utterly absurd;indeed,in view of the silliness of the majority of mankind,a wide-spread belief is more likely to be foolish than sensible.”Bertrand R

28、ussellSB战胜天才的办法：战胜天才的办法：SB可以随时叫可以随时叫100个个SB来证明来证明SB是对的，而天才怎么也叫不是对的，而天才怎么也叫不到到100个天才来证明天才是正确的。个天才来证明天才是正确的。&Appeal to Pity(诉诸诉诸怜悯怜悯)DefinitionDefinition&The argument tries to convince the audience of something by arousing its sense of pity and mercy.&This is most commonly used at law courts.&The fact

29、 that someone is pitiful does not make his/her wrong deeds right.ExampleExample&Im quite sure I was driving below the speed limit,officer;I had my eye on the speedometer.Ive had tickets before,and if you give me one now,it will probably cost my drivers licenseand if I lose my license Ill lose my job

30、.Dont you think a warning would be enough?Why didnt you drive slowlier at the first place?可怜之人必有可恨之处。可怜之人必有可恨之处。ExampleExample&The absurdity of appeal to pity is best ridiculed by a joke:a teenager is accused of murdering his father and mother with an axe;confronted with overwhelming proof of his gu

31、ilt,he pleaded for leniency on the grounds that he is an orphan.&Appeal to force(诉诸武力)DefinitionDefinition&The argument coerces the audience into accepting the conclusion by resorting to threat.&The famous proponent of this fallacy is Thrasymachus,for whom“might is right.”&The appeal to force is the

32、 abandonment of reason.秀才遇到兵，有理说不清。秀才遇到兵，有理说不清。ExampleExample“Socrates,I think that you are too ready to speak evil of men:and if you will take my advice,I would recommend you to be careful.Perhaps there is no city in which it is not easier to do men harm than to do them good,and this is certainly t

33、he case at Athens,as I believe that you know.”Plato,Meno&False Dichotomy 错误二分法错误二分法 DefinitionDefinition&In false dichotomy,the arguer sets up the situation so it looks like there are only two choices.The arguer then eliminates one of the choices,so it seems that we are left with only one option:the

34、 one the arguer wanted us to pick in the first place.But often there are really many different options,not just two.ExampleExample&Caldwell Hall is in a bad shape.Either we tear it down and put up a new building,or we continue to risk students safety.Obviously we shouldnt risk anyones safety,so we m

35、ust tear the building down.TipTip&Examine your own arguments:If youre saying that we have to choose between just two options,is that really so?Or are there other alternatives you havent mentioned?&If there are other alternatives,dont just ignore themexplain why they,too,should be ruled out.&Although

36、 theres no formal name for it,assuming that there are only three options,four options,etc.when really there are more is similar to false dichotomy and should also be avoided.&Begging the question 乞求窃取论点乞求窃取论点DefinitionDefinition&A complicated fallacy,it comes in several forms and can be hard to dete

37、ct.Basically,an argument that begs the question assumes the truth that it tries to prove,in the effort to prove it.It is circular.&The argument either relies on a premise that says the same thing as the conclusion or reasons in a circular way.&simplified formsA,therefore A.(A,therefore B.)B,therefor

38、e A.Example 1 Example 1&Active euthanasia is morally acceptable.It is a decent,ethical thing to help another human being escape suffering through death.Premise:It is a decent,ethical thing to help another human being escape suffering through death.Conclusion:Active euthanasia is morally acceptable.&

39、If we translate the premise,well see that the arguer has really just said the same thing twice:decent,ethical means pretty much the same thing as morally acceptable,and help another human being escape suffering through death means active euthanasia.&So the argument is really saying“Active euthanasia

40、 is morally acceptable because active euthanasia is morally acceptable.”It begs the question.More ExamplesMore Examples&To allow every man unbounded freedom of speech must always be,on the whole,advanta-geous to the state;for it is highly conducive to the interests of the community that each individ

41、ual should enjoy a liberty,perfectly unlimited,of expressing his sentiments.&God exists because this is what the Bible says,and the Bible is reliable because it is the words of God.&I am an honest man because Frank says so.How can we trust Frank?Simple.I will vouch for him.&the story of Hume&Complex

42、 QuestionDefinitionDefinition&A complex question is a question which presupposes the truth of some assumption(s)buried in that question.ExampleExample&“Why are private enterprises much more efficient than government-owned enterprises?”assumption:private enterprises are much more efficient than any g

43、overnment-owned enterprisesExampleExample&Complex questions are often used at court by shrewd lawyers to mislead the questioned person.“The figures seem to indicate that your sales increased as a result of these misleading advertisements.Is that correct?”“They did not!”“But you do admit,then,that yo

44、ur advertising was misleading.How long have you been engaging in practices like this?”When the question is complex,and its presuppositions are to be denied,they must be denied individually.ExampleExample&Are you in favor of increased governmental service and higher taxes?If you are,those whose taxes

45、 are already too high will vote against you.If you are not,those who need more services from the government will vote against you.In no case can you hope to win the election.&Ad hominem/against the person(人身攻击的谬误人身攻击的谬误)DefinitionDefinition&The ad hominem(against the person)fallacies focus our atten

46、tion on people rather than on arguments or evidence.In an ad hominem argument,the arguer attacks his or her opponent instead of the opponents argument.&There are two forms of arguments ad hominem.abusive ad hominemcircumstantial ad hominemabusive abusive ad ad hominemhominem&Abusive ad hominem is la

47、unched directly against persons,seeking to discredit or defame them.&That the arguer has a bad character does not necessarily mean the argument is bad.ExampleExample&Andrea Dworkin has written several books arguing that pornography harms women.But Dworkin is an ugly,bitter person,so you shouldnt lis

48、ten to her.circumstantial circumstantial ad ad hominemhominem&When the attack is indirectly against the persons,suggesting that they adopt their view chiefly because of their special circumstances or interests,it is called a“circumstantial ad hominem.”&That someone has adopted a view because of his/

49、her special circumstances or interests does not necessarily mean that view is wrong.ExamplesExamples&Im afraid,Mr.Black,that you are showing tolerance to homosexual marriage only because you want to win more public support for your campaign.“You are hurt.You should quit the game and go to the doctor

50、.”“Yeah,so that you can be the champion.”&Irrelevant Conclusion/non sequiurDefinitionDefinition&The premises of an argument do not support the conclusion.&In other words,the conclusion does not follow from the premises;or,we can say,the premises“miss the point.”ExampleExample&The seriousness of a pu