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1、Logical FallaciesLogical Fallacies逻辑逻辑谬误谬误Arguments,likemen,areoftenpretenders.PlatoWhat are fallacies?What are fallacies?&Fallaciesaredefectsthatweakenarguments.&Bylearningtolookfortheminyourownandotherswriting,youcanstrengthenyourabilitytoevaluatetheargumentsyoumake,read,andhear.two special notes
2、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 different kind
3、s of fallacies with different names.&ArgumentfromIgnoranceDefinitionDefinition&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 of prop
4、ositions; 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 exis
5、ts.&the story of Galileo&HastyGeneralizationDefinitionDefinition& 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.) a
6、re common 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 fathe
7、r was a smoker 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.
8、&Notice that in the 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 acciden
9、t.Almost every good rule 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
10、rational animal.Man is a 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
11、 A, A caused B&Of course, sometimes 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 r
12、elated as cause and effect. That is, 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, bu
13、t the argument hasnt shown us that one caused 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 i
14、ncrease is supposed to have produced higher 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/FalseAnalogy(错误类比错误类比)DefinitionDefinition
15、&Manyargumentsrelyonananalogybetweentwoormoreobjects,ideas,orsituations.Ifthethingsthatarebeingcomparedarentreallyalikeintherelevantrespects,theanalogyisaweakone,andtheargumentthatreliesonitcommitsthefallacyofweakanalogy.ExampleExample&Guns are like hammerstheyre both tools with metal parts that cou
16、ld be 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 no
17、t the ones at 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 o
18、n it. ExamplesExamples&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
19、 them work, the same must be 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.&Appealtoinappropriateauthority(诉诸诉诸不当不当权威权威)Defin
20、itionDefinition&Often we add 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 impressi
21、ng them with a famous name or by 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 th
22、eir opposition to it.&The most blatant 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;se
23、cond, rather than just say Dr. Authority believes 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
24、perceived as biased. &Adpopulum/AppealtoPeople(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
25、 fallacy (主流思想谬误), in which the arguer tries to 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
26、Pontiac Grand Prix? It could be that so many people are 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
27、will like you better if you believe it, and so forth. Keep 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 b
28、elief is more likely to be foolish than sensible.”Bertrand RussellSB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。&AppealtoPity(诉诸诉诸怜悯怜悯)DefinitionDefinition&Theargumenttriestoconvincetheaudienceofsomethingbyarousingitssenseofpityandmercy.&Thisismo
29、stcommonlyusedatlawcourts.&Thefactthatsomeoneispitifuldoesnotmakehis/herwrongdeedsright.ExampleExample&ImquitesureIwasdrivingbelowthespeedlimit,officer;Ihadmyeyeonthespeedometer.Ivehadticketsbefore,andifyougivemeonenow,itwillprobablycostmydriverslicenseandifIlosemylicenseIlllosemyjob.Dontyouthinkawa
30、rningwouldbeenough?Whydidntyoudriveslowlieratthefirstplace?可怜之人必有可恨之处。可怜之人必有可恨之处。ExampleExample&Theabsurdityofappealtopityisbestridiculedbyajoke:ateenagerisaccusedofmurderinghisfatherandmotherwithanaxe;confrontedwithoverwhelmingproofofhisguilt,hepleadedforleniencyonthegroundsthatheisanorphan.&Appeal
31、 to force(诉诸武力)DefinitionDefinition&Theargumentcoercestheaudienceintoacceptingtheconclusionbyresortingtothreat.&ThefamousproponentofthisfallacyisThrasymachus,forwhom“mightisright.”&Theappealtoforceistheabandonmentofreason.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 read
32、y 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 the case at Athens, as I believe that you know.”Plato, Meno&FalseDichotomy错误二分法错误二分法DefinitionDef
33、inition&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 one the arguer wanted us to pick in the first place. But often there are really many differe
34、nt 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 must tear the building down.TipTip&Examine your own arguments: If youre saying that we h
35、ave 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 theres no formal name for it, assuming that there are only three options, four
36、 options, etc. when really there are more is similar to false dichotomy and should also be avoided.&Beggingthequestion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 detect. Basically, an argument that begs the question assumes the truth that it t
37、ries 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, therefore A.Example 1 Example 1 &Activeeuthanasiaismorallyacceptable.Itisade
38、cent,ethicalthingtohelpanotherhumanbeingescapesufferingthroughdeath.Premise:Itisadecent,ethicalthingtohelpanotherhumanbeingescapesufferingthroughdeath.Conclusion:Activeeuthanasiaismorallyacceptable.&If we translate the premise, well see that the arguer has really just said the same thing twice: dece
39、nt, 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 is morally acceptable because active euthanasia is morally acceptable.” It begs the question.M
40、ore 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 individual should enjoy a liberty, perfectly unlimited, of expressing his sentiments.&God exists b
41、ecause this is what the Bible says, and the Bible is reliable because it is the words of God.&IamanhonestmanbecauseFranksaysso.HowcanwetrustFrank?Simple.Iwillvouchforhim.&thestoryofHume&ComplexQuestionDefinitionDefinition&A complex question is a question which presupposes the truth of some assumptio
42、n(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 government-owned enterprisesExampleExample&Complex questions are often used at court by shrewd lawyers to m
43、islead 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 your advertising was misleading. How long have you been engaging in practices like this?”Whenthequestioni
44、scomplex,anditspresuppositionsaretobedenied,theymustbedeniedindividually.ExampleExample&Are you in favor of increased governmental service and higher taxes? If you are, those whose taxes are already too high will vote against you. If you are not, those who need more services from the government will
45、 vote against you. In no case can you hope to win the election.&Adhominem/againsttheperson(人身攻击的谬误人身攻击的谬误)DefinitionDefinition&The ad hominem (against the person) fallacies focus our attention on people rather than on arguments or evidence. In an ad hominem argument, the arguer attacks his or her op
46、ponent instead of the opponents argument. &There are two forms of arguments ad hominem.abusive ad hominemcircumstantial ad hominemabusive abusive ad hominemad hominem&Abusive ad hominem is launched directly against persons, seeking to discredit or defame them.&That the arguer has a bad character doe
47、s 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 listen to her. circumstantial circumstantial ad hominemad hominem&When the attack is indirectly against the
48、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/her special circumstances or interests does not necessarily mean that view is wrong.ExamplesExamples&
49、Imafraid,Mr.Black,thatyouareshowingtolerancetohomosexualmarriageonlybecauseyouwanttowinmorepublicsupportforyourcampaign.“Youarehurt.Youshouldquitthegameandgotothedoctor.”“Yeah,sothatyoucanbethechampion.”&IrrelevantConclusion/non sequiurDefinitionDefinition&The premises of an argument do not support
50、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 punishment should match the seriousness of the crime. Right now, the punishment for drunk driving may simply be a fine. But drunk drivin
51、g is a very serious crime that can kill innocent people. So the death penalty should be the punishment for drunk driving. ExampleExample&All children should have ample attention from their parents. Parents who work full-time cannot give ample attention to their children. Therefore, mothers should no
52、t work full-time.&FallacyofCompositionDefinitionDefinition&two types of fallacies of compositionfirst type: reasoning from the attributes of the parts of the whole to the attributes of the whole itselfsecond type: reasoning from the attributes of the members of a collection to the attributes of the
53、collection itselfExamples: first typeExamples: first type&Since every part of a certain machine is light in weight, the machine “as a whole” is light in weight.&Since every scene in a play is a model of artistic perfection, the play as a whole is artistically perfect.&Since every player in the team
54、is good, the team as a whole is good.&The whole does not necessarily partake of the attributes of the individual parts because the problem of organization and structure is involved.你的眉毛、眼睛、耳朵、鼻子、嘴巴都长得你的眉毛、眼睛、耳朵、鼻子、嘴巴都长得很漂亮，但是一起放在你的脸上就不对了。很漂亮，但是一起放在你的脸上就不对了。Examples: second typeExamples: second type&
55、Since a bus uses more gasoline than a car, all buses use more gasoline than all cars.&Since a college students may enroll in no more than six courses each semester, all college students may enroll in no more than six courses each semester.&The second type is very confusing, because phrases such as “
56、all college students” can be used both “distributively,” meaning “all individual college students,” and “collectively,” meaning “all college students as a group.” &In the previous examples, if such phrases are use “distributively,” the conclusion is true; if “collectively,” false.cf. the first type
57、and the second typecf. the first type and the second type&The first type focuses on the relationship between the part and the whole, while the second type on the relationship between the individual member and the group.first type: a brick and a housesecond type: a brick and all bricks&FallacyofDivis
58、ionDefinitionDefinition&two types of fallacies of divisionfirst type: reasoning from the attributes of the whole to the attributes of the parts second type: reasoning from the attributes of the collection to the attributes of the members of the collection&It is the opposite of fallacy of composition
59、.Examples: first typeExamples: first type&Since America is a rich country and Johnson is an American citizen, Johnson must be rich.&Since the dormitory building is big and Lee is living in one of the rooms in the dormitory, Lee must be living in a big room. Examples: second typeExamples: second type
60、&The second type is very confusing, because sometimes they look quite similar to valid syllogisms.Humans are mortal.Socrates is a human.Therefore Socrates is mortal.Dogs are frequently encountered in the streets.Afghan hounds are dogs.Therefore Afghan hounds are frequently encountered in the streets
61、.Whats wrong?Whats wrong?&In the first example, “humans” are used “distributively,” meaning “all individual humans,” in the second example “dogs” are used “collectively,” meaning “all dogs as a group.” &In valid syllogisms, the terms should be used “distributively.” American Indians are disappearing.Rita is an American Indian.Therefore Rita is disappearing.This argument is invalid because here “American Indians” is used “collectively.”结束束语谢谢大家聆听！大家聆听！81
