Logical Fallacies逻辑谬误.ppt

Logic(partfour),Fallacies,Arguments,likemen,areoftenpretenders.Plato,Whatarefallacies?,Fallaciesaredefectsthatweakenarguments.Bylearningtolookfortheminyourownandotherswriting,youcanstrengthenyourabilitytoevaluatetheargumentsyoumake,read,andhear.,twospecialnotesaboutfallacies,Fallaciousargumentsarevery,verycommonandcanbequitepersuasive,atleasttothecasualreaderorlistener.Youcanfinddozensofexamplesoffallaciousreasoninginnewspapers,advertisements,andothersources.Therearemanydifferentkindsoffallacieswithdifferentnames.,ArgumentfromIgnorance,Definition,ArgumentfromIgnoranceassertsthatapropositionistruebecauseithasnotbeenprovedfalse,orthatitisfalsebecauseithasnotbeenprovedtrue.Inscienceandlogic,wemustverifythetruthorfalsehoodofpropositions;beforethat,nothingcanbeasserted.Injurisprudence,however,thereisanexception:apersonisalwaysregardedasinnocentbeforehe/sheisprovedguilty.,Examples,YoucantprovethatGodexists.Thereforehedoesntexist.YoucantprovethatGoddoesntexist.Thereforeheexists.thestoryofGalileo,HastyGeneralization,Definition,makingassumptionsaboutawholegrouporrangeofcasesbasedonasamplethatisinadequate(usuallybecauseitisatypicalorjusttoosmall).Stereotypesaboutpeople(“pilotstudentsarehandsome,gradstudentsarenerdy,etc.)arecommonexamplesofhastygeneralization.,Example,Myroommatesaidherphilosophyclasswashard,andtheoneIminishard,too.Allphilosophyclassesmustbehard!Twopersonsexperiencesare,inthiscase,notenoughonwhichtobaseaconclusion.Smokingcausescancerbecausemyfatherwasasmokerandhediedoflungcancer.,Tip,Askyourselfwhatkindofsampleyoureusing:Areyourelyingontheopinionsorexperiencesofjustafewpeople,oryourownexperienceinjustafewsituations?Ifso,considerwhetheryouneedmoreevidence,orperhapsalesssweepingconclusion.Noticethatinthepreviousexample,themoremodestconclusionSomephilosophyclassesarehardforsomestudentswouldnotbeahastygeneralization.,Accident,definition,Whenweapplyageneralizationtoindividualcasesthatitdoesnotproperlygovern,wecommitthefallacyofaccident.Almosteverygoodrulehasappropriateexceptions;wearguefallaciouslywhenwereasononthesuppositionthatsomerulesapplywithuniversalforce.Accidentistheoppositeoffalsegeneralization.,Example,Iamsuretheirambassadorwillbereasonableaboutthematter.Afterall,manisarationalanimal.Manisarationalanimal.Thatstrue,becausewehavereason.Butthatdoesnotmeanallmenbehaverationallyunderallcircumstances.,“Allgeneralizationsarefalse,includingthisone.”MarkTwain,Falsecause/Posthoc(假性因果),Definition,assumingthatbecauseBcomesafterA,AcausedBOfcourse,sometimesoneeventreallydoescauseanotheronethatcomeslater.forexample,ifIregisterforaclass,andmynamelaterappearsontheroll,itstruethatthefirsteventcausedtheonethatcamelater.Butsometimestwoeventsthatseemrelatedintimearentreallyrelatedascauseandeffect.Thatis,correlationisntthesamethingascausation.,Examples,PresidentJonesraisedtaxes,andthentherateofviolentcrimewentup.Jonesisresponsiblefortheriseincrime.Theincreaseintaxesmightormightnotbeonefactorintherisingcrimerates,buttheargumenthasntshownusthatonecausedtheother.Afloodhappenedafterthecometappeared;therefore,thecometcausedtheflood.misunderstandingofthelunareclipse,Tips,Toavoidtheposthocfallacy,thearguerwouldneedtogiveussomeexplanationoftheprocessbywhichthetaxincreaseissupposedtohaveproducedhighercrimerates.Thatswhatyoushoulddotoavoidcommittingthisfallacy:IfyousaythatAcausesB,youshouldhavesomethingmoretosayabouthowAcausedBthanjustthatAcomesfirstandBcomeslater!,Weak/FalseAnalogy(错误类比),Definition,Manyargumentsrelyonananalogybetweentwoormoreobjects,ideas,orsituations.Ifthethingsthatarebeingcomparedarentreallyalikeintherelevantrespects,theanalogyisaweakone,andtheargumentthatreliesonitcommitsthefallacyofweakanalogy.,Example,Gunsarelikehammerstheyrebothtoolswithmetalpartsthatcouldbeusedtokillsomeone.Andyetitwouldberidiculoustorestrictthepurchaseofhammerssorestrictionsonpurchasinggunsareequallyridiculous.Whilegunsandhammersdosharecertainfeatures,thesefeatures(havingmetalparts,beingtools,andbeingpotentiallyusefulforviolence)arenottheonesatstakeindecidingwhethertorestrictguns.Rather,werestrictgunsbecausetheycaneasilybeusedtokilllargenumbersofpeopleatadistance.Thisisafeaturehammersdonotshareitdbehardtokillacrowdwithahammer.Thus,theanalogyisweak,andsoistheargumentbasedonit.,Examples,Mindandrivers,canbebothbroad.Itisaknownfactthatthebroadertheriver,theshalloweritis.Thereforeitmustbetrue,thatthebroaderthemindis,theshalloweritis.Employeesandnailsarethesame.Asitisnecessarytohitnailsontheheadinordertomakethemwork,thesamemustbedonewithemployees.,Tip,Identifywhatproperties/attributesareimportanttotheclaimyouremaking,andseewhetherthetwothingsyourecomparingbothsharethoseproperties.Thisshowstheimportanceofrelevance.,Appealtoinappropriateauthority(诉诸不当权威),Definition,Oftenweaddstrengthtoourargumentsbyrefer-ringtorespectedsourcesorauthoritiesandexplainingtheirpositionsontheissueswerediscussing.Thisisquiteproperandevenencouragedinacademicresearch.If,however,wetrytogetreaderstoagreewithussimplybyimpressingthemwithafamousnameorbyappealingtoanauthoritywhohasnospecialcompetenceinthesphereconcerned,wecommitthefallacyofappealtoinappropriateauthority.,Example,Weshouldabolishthedeathpenalty.Manyrespectedpeople,suchasactorMr.Handsome,havepubliclystatedtheiroppositiontoit.Themostblatantexamplesofmisplacedappealstoauthorityappearinadvertising“testimonials.”,Tips,Therearetwoeasywaystoavoidcommittingappealtoinappropriateauthority:first,makesurethattheauthoritiesyouciteareexpertsonthesubjectyourediscussing;second,ratherthanjustsayDr.Authoritybelievesx,soweshouldbelieveit,too,trytoexplainthereasoningorevidencethattheauthorityusedtoarriveathisorheropinion.Italsohelpstochooseauthoritieswhoareperceivedasfairlyneutralorreasonable,ratherthanpeoplewhowillbeperceivedasbiased.,Adpopulum/AppealtoPeople(诉诸群众),Definition,Thearguertakesadvantageofthedesiremostpeoplehavetobelikedandtofitinwithothersandusesthatdesiretotrytogettheaudiencetoaccepthisorherargument.Oneofthemostcommonversionsisthebandwagonfallacy(主流思想谬误),inwhichthearguertriestoconvincetheaudiencetodoorbelievesomethingbecauseeveryoneelse(supposedly)does.Itisalsoverycommonlyusedinadvertisements.,Example,Gaymarriagesarejustimmoral.70%ofAmericansthinkso.WhyaresomanypeopleattractedbythePontiacGrandPrix?ItcouldbethatsomanypeopleareattractedbytheGrandPrixbecausesomanypeopleareattractedbytheGrandPrix!,Tip,Makesurethatyouarentrecommendingthatyouraudiencebelieveyourconclusionbecauseeveryoneelsebelievesit,allthecoolpeoplebelieveit,peoplewilllikeyoubetterifyoubelieveit,andsoforth.Keepinmindthatthepopularopinionisnotalwaystherightone!,“Thefactthatanopinionhasbeenwidelyheldisnoevidencewhatsoeverthatitisnotutterlyabsurd;indeed,inviewofthesillinessofthemajorityofmankind,awide-spreadbeliefismorelikelytobefoolishthansensible.”BertrandRussell,SB战胜天才的办法：SB可以随时叫100个SB来证明SB是对的，而天才怎么也叫不到100个天才来证明天才是正确的。,AppealtoPity(诉诸怜悯),Definition,Theargumenttriestoconvincetheaudienceofsomethingbyarousingitssenseofpityandmercy.Thisismostcommonlyusedatlawcourts.Thefactthatsomeoneispitifuldoesnotmakehis/herwrongdeedsright.,Example,ImquitesureIwasdrivingbelowthespeedlimit,officer;Ihadmyeyeonthespeedometer.Ivehadticketsbefore,andifyougivemeonenow,itwillprobablycostmydriverslicenseandifIlosemylicenseIlllosemyjob.Dontyouthinkawarningwouldbeenough?Whydidntyoudriveslowlieratthefirstplace?,可怜之人必有可恨之处。,Example,Theabsurdityofappealtopityisbestridiculedbyajoke:ateenagerisaccusedofmurderinghisfatherandmotherwithanaxe;confrontedwithoverwhelmingproofofhisguilt,hepleadedforleniencyonthegroundsthatheisanorphan.,Appealtoforce(诉诸武力),Definition,Theargumentcoercestheaudienceintoacceptingtheconclusionbyresortingtothreat.ThefamousproponentofthisfallacyisThrasymachus,forwhom“mightisright.”Theappealtoforceistheabandonmentofreason.,秀才遇到兵，有理说不清。,Example,“Socrates,Ithinkthatyouaretooreadytospeakevilofmen:andifyouwilltakemyadvice,Iwouldrecommendyoutobecareful.Perhapsthereisnocityinwhichitisnoteasiertodomenharmthantodothemgood,andthisiscertainlythecaseatAthens,asIbelievethatyouknow.”Plato,Meno,FalseDichotomy错误二分法,Definition,Infalsedichotomy,thearguersetsupthesituationsoitlookslikethereareonlytwochoices.Thearguertheneliminatesoneofthechoices,soitseemsthatweareleftwithonlyoneoption:theonethearguerwantedustopickinthefirstplace.Butoftentherearereallymanydifferentoptions,notjusttwo.,Example,CaldwellHallisinabadshape.Eitherwetearitdownandputupanewbuilding,orwecontinuetoriskstudentssafety.Obviouslyweshouldntriskanyonessafety,sowemusttearthebuildingdown.,Tip,Examineyourownarguments:Ifyouresayingthatwehavetochoosebetweenjusttwooptions,isthatreallyso?Orarethereotheralternativesyouhaventmentioned?Ifthereareotheralternatives,dontjustignorethemexplainwhythey,too,shouldberuledout.Althoughtheresnoformalnameforit,assumingthatthereareonlythreeoptions,fouroptions,etc.whenreallytherearemoreissimilartofalsedichotomyandshouldalsobeavoided.,Beggingthequestion乞求窃取论点,Definition,Acomplicatedfallacy,itcomesinseveralformsandcanbehardtodetect.Basically,anargumentthatbegsthequestionassumesthetruththatittriestoprove,intheefforttoproveit.Itiscircular.Theargumenteitherreliesonapremisethatsaysthesamethingastheconclusionorreasonsinacircularway.simplifiedformsA,thereforeA.(A,thereforeB.)B,thereforeA.,Example1,Activeeuthanasiaismorallyacceptable.Itisadecent,ethicalthingtohelpanotherhumanbeingescapesufferingthroughdeath.Premise:Itisadecent,ethicalthingtohelpanotherhumanbeingescapesufferingthroughdeath.Conclusion:Activeeuthanasiaismorallyacceptable.,Ifwetranslatethepremise,wellseethatthearguerhasreallyjustsaidthesamethingtwice:decent,ethicalmeansprettymuchthesamethingasmorallyacceptable,andhelpanotherhumanbeingescapesufferingthroughdeathmeansactiveeuthanasia.Sotheargumentisreallysaying“Activeeuthanasiaismorallyacceptablebecauseactiveeuthanasiaismorallyacceptable.”Itbegsthequestion.,MoreExamples,Toalloweverymanunboundedfreedomofspeechmustalwaysbe,onthewhole,advanta-geoustothestate;foritishighlyconducivetotheinterestsofthecommunitythateachindividualshouldenjoyaliberty,perfectlyunlimited,ofexpressinghissentiments.GodexistsbecausethisiswhattheBiblesays,andtheBibleisreliablebecauseitisthewordsofGod.,IamanhonestmanbecauseFranksaysso.HowcanwetrustFrank?Simple.Iwillvouchforhim.thestoryofHume,ComplexQuestion,Definition,Acomplexquestionisaquestionwhichpresupposesthetruthofsomeassumption(s)buriedinthatquestion.,Example,“Whyareprivateenterprisesmuchmoreefficientthangovernment-ownedenterprises?”assumption:privateenterprisesaremuchmoreefficientthananygovernment-ownedenterprises,Example,Complexquestionsareoftenusedatcourtbyshrewdlawyerstomisleadthequestionedperson.,“Thefiguresseemtoindicatethatyoursalesincreasedasaresultofthesemisleadingadvertisements.Isthatcorrect?”“Theydidnot!”“Butyoudoadmit,then,thatyouradvertisingwasmisleading.Howlonghaveyoubeenengaginginpracticeslikethis?”,Whenthequestioniscomplex,anditspresuppositionsaretobedenied,theymustbedeniedindividually.,Example,Areyouinfavorofincreasedgovernmentalserviceandhighertaxes?Ifyouare,thosewhosetaxesarealreadytoohighwillvoteagainstyou.Ifyouarenot,thosewhoneedmoreservicesfromthegovernmentwillvoteagainstyou.Innocasecanyouhopetowintheelection.,Adhominem/againsttheperson(人身攻击的谬误),Definition,Theadhominem(againsttheperson)fallaciesfocusourattentiononpeopleratherthanonargumentsorevidence.Inanadhominemargument,thearguerattackshisorheropponentinsteadoftheopponentsargument.Therearetwoformsofargumentsadhominem.abusiveadhominemcircumstantialadhominem,abusiveadhominem,Abusiveadhominemislauncheddirectlyagainstpersons,seekingtodiscreditordefamethem.Thatthearguerhasabadcharacterdoesnotnecessarilymeantheargumentisbad.,Example,AndreaDworkinhaswrittenseveralbooksarguingthatpornographyharmswomen.ButDworkinisanugly,bitterperson,soyoushouldntlistentoher.,circumstantialadhominem,Whentheattackisindirectlyagainstthepersons,suggestingthattheyadopttheirviewchieflybecauseoftheirspecialcircumstancesorinterests,itiscalleda“circumstantialadhominem.”Thatsomeonehasadoptedaviewbecauseofhis/herspecialcircumstancesorinterestsdoesnotnecessarilymeanthatviewiswrong.,Examples,Imafraid,Mr.Black,thatyouareshowingtolerancetohomosexualmarriageonlybecauseyouwanttowinmorepublicsupportforyourcampaign.,“Youarehurt.Youshouldquitthegameandgotothedoctor.”“Yeah,sothatyoucanbethechampion.”,IrrelevantConclusion/nonsequiur,Definition,Thepremisesofanargumentdonotsupporttheconclusion.Inotherwords,theconclusiondoesnotfollowfromthepremises;or,wecansay,thepremises“missthepoint.”,Example,Theseriousnessofapunishmentshouldmatchtheseriousnessofthecrime.Rightnow,thepunishmentfordrunkdrivingmaysimplybeafine.Butdrunkdrivingisaveryseriouscrimethatcankillinnocentpeople.Sothedeathpenaltyshouldbethepunishmentfordrunkdriving.,Example,Allchildrenshouldhaveampleattentionfromtheirparents.Parentswhoworkfull-timecannotgiveampleattentiontotheirchildren.Therefore,mothersshouldnotworkfull-time.,FallacyofComposition,Definition,twotypesoffallaciesofcompositionfirsttype:reasoningfromtheattributesofthepartsofthewholetotheattributesofthewholeitselfsecondtype:reasoningfromtheattributesofthemembersofacollectiontotheattributesofthecollectionitself,Examples:firsttype,Sinceeverypartofacertainmachineislightinweight,themachine“asawhole”islightinweight.Sinceeverysceneinaplayisamodelofartisticperfection,theplayasawholeisartisticallyperfect.Sinceeveryplayerintheteamisgood,theteamasawholeisgood.,Thewholedoesnotnecessarilypartakeoftheattributesoftheindividualpartsbecausetheproblemoforganizationandstructureisinvolved.,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。,Examples:secondtype,Sinceabususesmoregasolinethanacar,allbusesusemoregasolinethanallcars.Sinceacollegestudentsmayenrollinnomorethansixcourseseachsemester,allcollegestudentsmayenrollinnomorethansixcourseseachsemester.,Thesecondtypeisveryconfusing,becausephrasessuchas“allcollegestudents”canbeusedboth“distributively,