Logical-Fallacies逻辑谬论.ppt

Logical-Fallacies逻辑谬论.ppt

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LogicalFallacies,ContinuingourforayintotheworldofArgument,Courtesyof:

http:

/www.unc.edu/depts/wcweb/handouts/fallacies.html,WhatisFallacy?

Fallaciesaredefectsthatweakenarguments.First,fallaciousargumentsarevery,verycommonandcanbequitepersuasive,atleasttothecausalreaderorlistener.Youcanfinddozensofexamplesoffallaciousreasoninginnewspapers,advertisements,andothersources.Second,itissometimeshardtoevaluatewhetheranargumentisfallacious.Anargumentmightbeveryweak,somewhatweak,somewhatstrong,orverystrong.Anargumentthathasseveralstagesorpartsmighthavesomestrongsectionsandsomeweakones.,HastyGeneralization,Definition:

Makingassumptionsaboutawholegrouporrangeofcasesbasedonasamplethatisinadequate（usuallybecauseitisatypicalorjusttoosmall）.Stereotypesaboutpeople（fratboysaredrunkards,gradstudentsarenerdy,etc.）areacommonexampleoftheprincipleunderlyinghastygeneralization.Example:

Myroommatesaidherphilosophyclasswashard,andtheoneIminishard,too.Allphilosophyclassesmustbehard!

Twopeoplesexperiencesare,inthiscase,notenoughonwhichtobaseaconclusion.,MissingthePoint,Definition:

Thepremisesofanargumentdosupportaparticularconclusion-butnottheconclusionthattheargueractuallydraws.Example:

Theseriousnessofapunishmentshouldmatchtheseriousnessofthecrime.Rightnow,thepunishmentfordrunkdrivingmaysimplybeafine.Butdrunkdrivingisaveryseriouscrimethatcankillinnocentpeople.Sothedeathpenaltyshouldbethepunishmentfordrunkdriving.Theargumentactuallysupportsseveralconclusions-Thepunishmentfordrunkdrivingshouldbeveryserious,inparticular-butitdoesntsupporttheclaimthatthedeathpenalty,specifically,iswarranted.,Posthoc（falsecause）,ThisfallacygetsitsnamefromtheLatinphraseposthoc,ergopropterhoc,whichtranslatesasafterthis,thereforebecauseofthis.Definition:

AssumingthatbecauseBcomesafterA,AcausedB.Ofcourse,sometimesoneeventreallydoescauseanotheronethatcomeslater-forexample,ifIregisterforaclass,andmynamelaterappearsontheroll,itstruethatthefirsteventcausedtheonethatcamelater.Butsometimestwoeventsthatseemrelatedintimearentreallyrelatedascauseandevent.Thatis,correlationisntthesamethingascausation.Examples:

PresidentJonesraisedtaxes,andthentherateofviolentcrimewentup.Jonesisresponsiblefortheriseincrime.“Theincreaseintaxesmightormightnotbeonefactorintherisingcrimerates,buttheargumenthasntshownusthatonecausedtheother.,SlipperySlope,Definition:

Thearguerclaimsthatasortofchainreaction,usuallyendinginsomedireconsequence,willtakeplace,buttheresreallynotenoughevidenceforthatassumption.Thearguerassertsthatifwetakeevenonestepontotheslipperyslope,wewillendupslidingallthewaytothebottom;heorsheassumeswecantstophalfwaydownthehill.Example:

Animalexperimentationreducesourrespectforlife.Ifwedontrespectlife,wearelikelytobemoreandmoretolerantofviolentactslikewarandmurder.Soonoursocietywillbecomeabattlefieldinwhicheveryoneconstantlyfearsfortheirlives.Itwillbetheendofcivilization.Topreventthisterribleconsequence,weshouldmakeanimalexperimentationillegalrightnow.Sinceanimalexperimentationhasbeenlegalforsometimeandcivilizationhasnotyetended,itseemsparticularlyclearthatthischainofeventswontnecessarilytakeplace.,Alsoknownas“theCamelsNose,WeakAnalogy,Definition:

Manyargumentsrelyonananalogybetweentwoormoreobjects,ideas,orsituations.Ifthetwothingsthatarebeingcomparedarentreallyalikeintherelevantrespects,theanalogyisaweakone,andtheargumentthatreliesonitcommitsthefallacyofweakanalogy.Example:

Gunsarelikehammers-theyrebothtoolswithmetalpartsthatcouldbeusedtokillsomeone.Andyetitwouldberidiculoustorestrictthepurchaseofhammers-sorestrictionsonpurchasinggunsareequallyridiculous.Whilegunsandhammersdosharecertainfeatures,thesefeatures（havingmetalparts,beingtools,andbeingpotentiallyusefulforviolence）arenottheonesatstakeindecidingwhethertorestrictguns.Rather,werestrictgunsbecausetheycaneasilybeusedtokilllargenumbersofpeopleatadistance.Thisisafeaturehammersdonotshare-itdbehardtokillacrowdwithahammer.Thus,theanalogyisweak,andsoistheargumentbasedonit.Ifyouthinkaboutit,youcanmakeananalogyofsomekindbetweenalmostanytwothingsintheworld:

Mypaperislikeamudpuddlebecausetheybothgetbiggerwhenitrains（IworkmorewhenImstuckinside）andtheyrebothkindofmurky.Sothemerefactthatyoudrawananalogybetweentwothingsdoesntprovemuch,byitself.,AppealtoAuthority,Definition:

Oftenweaddstrengthtoourargumentsbyreferringtorespectedsourcesorauthoritiesandexplainingtheirpositionsontheissueswerediscussing.If,however,wetrytogetreaderstoagreewithussimplybyimpressingthemwithafamousnameorbyappealingtoasupposedauthoritywhoreallyisntmuchofanexpert,wecommitthefallacyofappealtoauthority.Example:

Weshouldabolishthedeathpenalty.Manyrespectedpeople,suchasactorGuyHandsome,havepubliclystatedtheiroppositiontoit.WhileGuyHandsomemaybeanauthorityonmattershavingtodowithacting,theresnoparticularreasonwhyanyoneshouldbemovedbyhispoliticalopinions-heisprobablynomoreofanauthorityonthedeathpenaltythanthepersonwritingthepaper.,AppealtoPity,Definition:

Theappealtopitytakesplacewhenanarguertriestogetpeopletoacceptaconclusionbymakingthemfeelsorryforsomeone.Example:

Iknowtheexamisgradedbasedonperformance,butyoushouldgivemeanA.Mycathasbeensick,mycarbrokedown,andIvehadacold,soitwasreallyhardformetostudy!

TheconclusionhereisYoushouldgivemeanA.ButthecriteriaforgettinganAhavetodowithlearningandapplyingthematerialfromthecourse;theprinciplethearguerwantsustoaccept（peoplewhohaveahardweekdeserveAs）isclearlyunacceptable.Example:

Itswrongtotaxcorporations-thinkofallthemoneytheygivetocharity,andofthecoststheyalreadypaytoruntheirbusinesses!

AppealtoIgnorance,Definition:

Intheappealtoignorance,thearguerbasicallysays,Look,theresnoconclusiveevidenceontheissueathand.Therefore,youshouldacceptmyconclusiononthisissue.Example:

PeoplehavebeentryingforcenturiestoprovethatGodexists.Butnoonehasyetbeenabletoproveit.Therefore,Goddoesnotexist.Heresanopposingargumentthatcommitsthesamefallacy:

PeoplehavebeentryingforyearstoprovethatGoddoesnotexist.Butnoonehasyetbeenabletoproveit.Therefore,Godexists.Ineachcase,thearguertriestousethelackofevidenceassupportforapositiveclaimaboutthetruthofaconclusion.Thereisonesituationinwhichdoingthisisnotfallacious:

Ifqualifiedresearchershaveusedwell-thought-outmethodstosearchforsomethingforalongtime,theyhaventfoundit,anditsthekindofthingpeopleoughttobeabletofind,thenthefactthattheyhaventfounditconstitutessomeevidencethatitdoesntexist.,StrawMan,Definition:

Onewayofmakingourownargumentsstrongeristoanticipateandrespondinadvancetotheargumentsthatanopponentmightmake.Thearguersetsupawimpyversionoftheopponentspositionandtriestoscorepointbyknockingitdown.Example:

Feministswanttobanallpornographyandpunisheveryonewhoreadsit!

Butsuchharshmeasuresaresurelyinappropriate,sothefeministsarewrong:

pornanditsreadersshouldbeleftinpeace.Thefeministargumentismadeweakbybeingoverstated-infact,mostfeministsdonotproposeanoutrightbanonpornoranypunishmentforthosewhomerelyreadit;often,theyproposesomerestrictionsonthingslikechildporn,orproposetoallowpeoplewhoarehurtbyporntosuepublishersandproducers,notreaders,fordamages.,RedHerring,Definition:

Partwaythroughanargument,thearguergoesoffonatangent,raisingasideissuethatdistractstheaudiencefromwhatsreallyatstake.Often,thearguerneverreturnstotheoriginalissue.Example:

Gradingthisexamonacurvewouldbethemostfairthingtodo.Afterall,classesgomoresmoothlywhenthestudentsandtheprofessoraregettingalongwell.Letstryourpremise-conclusionoutliningtoseewhatswrongwiththisargument:

Premise:

Classesgomoresmoothlywhenthestudentsandtheprofessoraregettingalongwell.Conclusion:

Gradingthisexamonacurvewouldbethemostfairthingtodo.Whenwelayitoutthisway,itsprettyobviousthatthearguerwentoffonatangent-thefactthatsomethinghelpspeoplegetalongdoesntnecessarilymakeitmorefair;fairnessandjusticesometimesrequireustodothingsthatcauseconflict.Buttheaudiencemayfeelliketheissueofteachersandstudentsagreeingisimportantandbedistractedfromthefactthatthearguerhasnotgivenanyevidenceastowhyacurvewouldbefair.,FalseDichotomy,Definition:

Infalsedichotomy,thearguersetsupthesituationsoitlookslikethereareonlytwochoices.Thearguertheneliminatesoneofthechoices,soitseemsthatweareleftwithonlyoneoption:

theonethearguerwantedustopickinthefirstplace.Example:

CaldwellHallisinbadshape.Eitherwetearitdownandputupanewbuilding,orwecontinuetoriskstudentssafety.Obviouslyweshouldntriskanyonessafety,sowemusttearthebuildingdown.Theargumentneglectstomentionthepossibilitythatwemightrepairthebuildingorfindsomewaytoprotectstudentsfromtherisksinquestion-forexample,ifonlyafewroomsareinbadshape,perhapsweshouldntholdclassesinthoserooms.,BeggingtheQuestion,Definition:

Acomplicate