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JonMcGinnis“ExperimentalthoughtsonthoughtexperimentsinmedievalIslam”AbstractThestudybeginswiththelanguageemployedinandthepsychologicalbasisofthoughtexperimentsasunderstoodbycertainmedievalArabicphilosophers.ItthenprovidesataxonomyofdifferentkindsofthoughtsexperimentsusedinthemedievalIslamicworld.Theseincludepurelyfictionalthoughtexperiments,idealizationsandfinallythoughtexperimentsusingingeniousmachines.Thestudyconcludesbysuggestingthatthoughtexperimentsprovidedahalfwayhouseduringthisperiodbetweenastaunchrationalismandanemergingempiricism.
1.Introduction
Thereisno(medieval)Arabictermorphrasefor“thoughtexperiment”.Be
thatasitmay,medievalphilosophersandscientistsworkinginArabicboth
concretelyemployedthoughtexperimentsintheirphilosophiesanddiscussedtheir
meritsanddemeritsabstractly.Indeed,itwouldseemthatthoughtexperiments
trulycapturedtheimaginationofmedievalthinkersintheMuslimworld,wholeft
behindasignificantbodyofexamplesandanalysesofsuchexperiments.What
followsmakesnopretensetobeingacompletehistoryofthatbodyofwork.Instead,
thisstudyfocusesprimarily,althoughbynomeansexclusively,onthought
experimentsastheyareusedanddiscussedinIbnSīnā(980–1037),theAvicennaof
Latinfame.Alongtheway,however,thisstudyalsotouchesonothernotablefigures
andtheirusesandthoughtsaboutthoughtexperiments.Thesefiguresincludethe
famedmedievalArabicopticianIbnal-Haytham(965–1040,Lt.Alhazen)andthe
renownedMuslimTheologianAbūḤamidal-Ghazālī(1058–1111).
Asafirstpass,onecandividethoughtexperimentsinthemedievalArabic
worldintotwoclasses:thosethatareinprincipleimpossibletocarryoutandthose
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thatatleastappeartobepossibletocarryoutevenifinpracticetheycannot.
Examplesfromcontemporaryphilosophyofthefirstclassincludezombieworlds
andpersonssplittingandrecombining,etc.Inthemedievalperiodjustasnowsuch
thoughtexperimentsfunctionedprimarilyasintuitionpumpsintendedtogive
someoneasensesofwhatisatleastpossible.Irefertothisclassofthought
experimentsas“fictionalthoughtexperiments”.Contemporaryexamplesofthe
secondclassofthoughtexperimentsaboundintheworksofEinstein,as,for
instance,ridingalightbeamorhisuseofmovingtrainsandlighteningflashesin
relationtosimultaneityorone’sexpectedexperienceinafreefallingelevatorto
explaingravity.Sincewithinthemedievalperiodthoughtexperimentsofthissort
frequentlydescribeidealizedaccountsofotherwiserealizablesituations,Ireferto
thisclassas“idealizedthoughtexperiments.”Theclassofidealizedthought
experimentsfurtherdividesintothosethatappealtosomeformofmechanical
apparatus,towhichthename“mechanicalthoughtexperiments”isappropriate,and
thosethatdonot.
Continuingthisfirstpass,medievalthinkersintheMuslimworldalsohad
differentaimsforthoughtexperiments.Insomecases,theaimwassimplytohelp
oneenvisionorvividlytograspsomeabstractconclusionofademonstration.As
such,thethoughtexperimentsarenotintegraltotheactualproof(s)forthedesired
conclusion.Inothercases,athoughtexperimentisintegraltotheproofinthatit
eitherconstitutesthewholeoftheargumentorisintendedtoestablishanecessary
premisefortheargument.Additionally,inthosecaseswherethethought
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experimentintendstoproveapremise,itmightshowthatsomestateofaffairsisat
leastpossibleormoresignificantlythatsomescenarioisfactive.
Towardsdevelopingthesesketchyremarks,Ibeginwithabriefdiscussion
aboutthelanguageandpsychologyofimagination,particularlyasitoccursinthe
worksofAvicenna.ThatAvicennainfactembeddedthoughtexperimentswithinan
overallpsychologyseemstosethimapartfromthethinkersthatprecededhim.
Followingthecommentsonpsychology,theremainderofthestudyisataxonomyof
varioussortsofthoughtexperimentsusedamongthinkersinthemedievalIslamic
world:first,instancesoffictionalonesandsecondidealizedones.Whenpossible,I
alsodiscussthephilosophicalattitudesandresponsestothevariousthought
experiments.WhatIhopeemergesisasketchoftheplaceofthoughtexperiments
amongmedievalArabic-speakingphilosophersandscientiststhatothersmayuseto
fillinthewholepicture.
2.TheLanguageandPsychologyofThoughtExperimentsintheMedievalMuslim
World1
WhilethereislittledoubtthatancientGreekphilosopherscraftedand
employedwhatwenowcall“thoughtexperiments,”(seeIerodiakonouandBecker
inthepresentvolume),KaterinaIerodiakonouhasalsonoted,“thereisnoevidence
that[theancients]classifiedexamplesbasedonimaginaryorinventedassumptions
inaspecialcategory”(seeIerodiakonouinthepresentvolume,p.xxx).Bythetime
1ThemostdetaileddiscussiontodateabouttherelationofthoughtexperimentstotheoriesofpsychologydevelopedinthemedievalArabicworldisTanelliKukkonen’slandmark2014article,towhichthissectionisheavilyindebted.
4
ofAvicenna,however,thepremisesdrivingthoughtexperimentswereseentoform
aspecialcategoryorattheveryleasttopresentaspecialproblem.Toappreciatethe
problemonemustbeginwiththelanguagethatAristotleandhiscommentators
usedforthoughtexperiments,foritisthatterminologythatmedievalArabic-
speakingphilosophersprimarilyinheritedandusedwhenconstructingor
discussingthoughtexperiments.
PerhapsmostfrequentlyAristotleintroducesthoughtexperimentswitha
conditionalstatement(SeeKukkonen2002and2014,esp.§I;andIerodiakonou
2005,esp.§IV).Theconditional’santecedentthenfunctionsasaninitialsupposition
thatgovernsthethoughtexperiment.Insomeplaces—likePhysics,7.1,242a9–10,
whereAristotleaguesagainstthepossibilityofself-motion—heexplicitly
introducesathoughtexperimentasahypothesisorsupposition(hupekeito).In
medievalArabic,oratleastinAvicenna’sphilosophicalvocabulary,thenotionofan
hypothesisorsupposition,particularlyasusedinthoughtexperiments,isusually
renderedbyfarḍ.
Additionally,Aristotleandhiscommentatorssometimesrefertothought
experimentsusingtermsderivedfromnoein,“tothink”.OneexampleinAristotleis
atPhysics,3.8,208a14–16,wherehecriticizescertainthoughtexperiments
involvinginfinity.Aristotle’slateNeoplatoniccommentator,JohnPhiloponus(490–
570),inhiscommentaryonthePhysics(Philoponus,InPhysicorum,574.14;575.8;
10;18)hasamoreapprovingappraisalofexperiments“inthought”(kat’epinoian)
whendefendingtheideaofanimmaterialextension.TheGreektermnoeinandits
cognateswerefrequentlyrenderedintoArabicwithsomeformofʿaql,“tointellect”.
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Theobjectsofintellect(maʿqūlāt)indicatetheuniversalessencesofthings
abstractedfromtheirmaterialconditions.Assuchtheobjectsofintellecthardly
seemsuitableincaseswhereparticularsarebeingimaginedorwhere
counterfactualpremisesareneeded.
Whatisneededinthesecasesaremorefantasticalimaginations,inGreek
phantasia.Thetransliterationfanṭāsīyāorthenativetermkhayālwasfrequently
usedinArabictocapturethenotionofphantasia.AmongmedievalArabic
Peripateticsbothfanṭāsīyāandkhayālwereusedtoindicateeitheraparticular
psychologicalfaculty,namely,imagination,ortheproductofsomeinternal
psychologicalfaculty.Therecognizeddifficultywithusingmereimaginationsor
fantasiesinthoughtsexperimentsisthatthereseemstobenocheckonthe
imaginativefacultytoensurethatitsobjectstellussomethinginformativeaboutthe
world.ItisjustsuchaconcernthatpromptedthelateHellenisticNeoplatonist,
Simplicius(c.490–c.560),tocomplainaboutputtingone’sfaithinsuchfantasies
(Simplicius,InDecaelo,418.30).
OneisnowinapositiontoseethespecialproblemthatAvicennaseemsto
recognizeaboutthepremisesusedinthoughtexperiments.Ifthesepremisesare
productsofthefacultyofintellect,then,asTaneliKukkonenacutelyobserves,they
“onlyidealizematerialcircumstancesintheframeworkofawell-definedsetof
assumednaturallawsandinvariance”(Kukkonen2014,446).Inotherwords,
premisesproducedbytheintellectdonotlendthemselvestothecounterfactual
scenariosthatfrequentlyareatthecoreofathoughtexperiments.Alternatively,if
thepremisesofthoughtexperimentsarenothingmorethanunbridledcompositions
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oftheimagination,thenthereisnoassurancethattheircontentconnectsupwith
anythingintheworldsoastogiveoneadeeperinsightintotheworld.ForAvicenna
thequestionatstakeisapsychologicalone:whatfacultyofthesoulproducesthe
premisesemployedin(legitimate)thoughtexperimentsasopposedtowildravings?
Intellectseemstoorestrictedandimaginationseemstoounrestrained.
Avicenna’ssolutiontothisdilemmawastointroduceanewinternalsensory
faculty,wahm,whichforlackofanyexactEnglishtranslationisusuallytermedthe
estimativefaculty.(FordiscussionsofAvicenna’stheoryofwahmseeBlack1993,
Hasse2000,esp.II.2,Hall2006,Kukkonen2014,esp.§3.)Avicennaidentifiesthe
estimativefacultyamongthefiveinternalperceptivefaculties,whicharecommonto
humansand(higher)non-humananimalsalike(AvicennaDeanima,4.3).These
facultiesincludecommonsense,memory,theretentiveandcompositive
imaginationsandfinallytheestimativefaculty.AccordingtoAvicenna,the
estimativefacultyperceivesnon-sensiblefeaturesorintentions(sing.maʿná)within
sensibleparticularthings.Theclassicexampleisthesheep’srecognitionofthe
particularferocityinagivenwolf,forwhileferocityisnotitselfsomethingsensible
itismanifestedinthesensiblefeaturesofthewolf,likeitssharpfangsandclaws
andthecarnivorousodorthatitexudes.Innon-humananimalstheestimative
facultyisthehighestfunctioningpsychologicalpower,lessthanintellectbutalso
morethanmereimagination.Itallowstheseanimalstointeractwiththeworld
aroundtheminafairlyaccurateway.Eveninhumans,accordingtoAvicenna,itis
theestimativefacultythatallowsustonavigatemanyofourday-to-day
interactions.
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Additionally,Avicennaappealstotheestimativefacultytoexplaintheobjects
andpremisesofthemathematicalsciences.Theobjectsofmathematics,Avicenna
tellsus,arecertainformalfeaturesofmaterialobjectsbutwhichcanbeconsidered
intheestimativefacultyasabstractedfromtheirmaterialconditions,like,for
example,squareness(Avicenna,Madkhal,1.2,12–13).Inthisrespect,theestimative
facultyiswhatallowsthemathematiciantoconsiderperfectgeometricalfiguresor
numbersintheabstracteventhoughtheseareneverinstantiatedphysically;itisthe
powerthatallowsthephysiciststoimagineperfectlyfrictionlessplanesora
sphere’stouchingatwo-dimensionalsurfaceatasinglepoint,eventhoughagainin
thenitty-grittyworldaroundusnoneoftheseexists.Thesemathematicalabstracta,
Avicennasays,existbysupposition(bi-l-farḍ),usuallyasuppositionimaginedbythe
estimativefaculty.Thatistosay,whilemathematicalabstractaexistinamentalact
ofconceptualization(taṣawwur),theydonotexists,atleastnotintheexactwaythat
themathematicianinvestigatesthem,intheconcretematerialparticularsthat
populatetheworld.Itistheestimativefaculty,then,thatprovidesmathematicians
and(theoretical)physicistswithanidealizedpictureoftheworld.Inthisrespect,
theestimativefacultyoffersuparoughandreadyguidetorealphysical
possibilities.Stillonemustbecarefultodistinguishbetweenwhatexistsassuchin
theestimativefacultyandwhatactuallyexistasseparateintheworld.ForAvicenna,
ifoneistomovefromthepossibilitiesimaginedintheestimativefacultytowhat
actuallyexists,onemustalsohaveademonstrationorprovidesomeactualinstance
intheworldofwhattheestimativefacultyposits.
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Tosumup,Avicennadevelopedthenotionofanestimativefacultyinorder
toexplainanumberofdisparate,albeitrelated,phenomena.Amongthese
phenomenaarethesemi-rationalthoughtsandcognitiveprocessesofthosehigher
animalsthatlackanintellect.Anotherwastoshowhowidealizationsusedin
mathematics,whichdonotactuallyexistseparatelyintheworld,canbeinformative
abouttheworld.Finally,theestimativefacultyprovidesAvicennawitha
psychologicalunderpinningforthoughtexperiments,whichdoesjusticetotheir
frequentlycounterfactualnaturewhilealsoexplaininghowtheycanhaveimport
abouttheworldasitactuallyis.
3.FictionalThoughtExperimentsintheMedievalIslamicWorld
InthissectionIconsidertwosortsoffictionalthoughtexperimentswithvery
differentaims.Inonecase,thethoughtexperimentfunctionsasasubsidiaryaidto
helponebettergrasptheconclusionofsomeargumentthatisindependentofthe
thoughtexperiment.Intheothercase,thethoughtexperimentisintegraltothe
overallargument.Againfictionalthoughtexperimentsproceedfromaninitial
suppositionthatisphysicallyimpossibleinprincipletocarryout,although
presumablyanall-powerfulagent,likeGod,couldrealizethescenario.Arguably,the
bestknownfictionalthoughtexperimentcomingfromthemedievalIslamicworldis
Avicenna’sfamous“flyingman”(seeMarmura1986,Druart1988,Hasse2000,esp.
II.1).HereisthatthoughtexperimentinAvicenna’sownwords:
9
Oneshouldimaginethroughanactoftheestimativefaculty(yatawahhama)
asifoneofuswerecreatedcompleteandperfectallatoncebuthissightis
veiledfromdirectlyobservingthethingsoftheexternalworld.Heiscreated
asthoughfloatinginairorinavoidbutwithouttheairsupportinghimsuch
thathewouldfeelit,andthelimbsofhisbodyarestretchedoutandaway
fromoneanother,sotheydonotcomeintocontactortouch.Thenhe
considerswhetherhecanasserttheexistenceofhisself.Hehasnodoubts
aboutassertinghisselfassomethingthatexistswithoutalso[havingto]
asserttheexistenceofanyofhisexteriororinteriorparts,hisheart,his
brain,oranythingexternal.(Avicenna,Deanima,1.1,16)
Avicennapresentsthisthoughtexperimentnofewerthanfivetimesthroughouthis
oeuvre.2Thepurposeofthethoughtexperimentistogetonetothinkofone’sself
(dhāt)asperhapsdistinctfromone’sbodyorsensibleapprehensions.Forit
certainlyseemspossible,evenifonlybyanactofGod,thatanindividualcouldcome
intoexistenceallatoncedevoidofanysensoryinput,sensationsorsensible
memories.Yeteveninthisdeprivedstatetheindividual,oneimagines,wouldbe
awareorconsciousofhisorherself(shuʿūrbi-dhāt),orsoAvicennaimagines.(Fora
detailedstudyofself-awareness(orconsciousness)inthethoughtofAvicennasee
JariKaukua2015.)
2TheseareinDeanima1.1(translatedhere)and5.7,theMashriqīyūn,Ishārātwa-l-tanbīhātandal-Risālaal-Aḍḥawīyafīl-maʿād.SeeHasse2000,80–7foradiscussionofthedifferencesamongthevariouspresentations.
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WhatisimportanttonoteisthatAvicennadoesnotclaimherethatthis
thoughtexperimentdemonstratesthatthehumansoulorselfisimmaterial.Whatis
neededtrulytoestablishthatconclusionisaproperdemonstration,whichAvicenna
providesinadditiontothethoughtexperiment(seeAvicenna,Deanima,5.2).
Instead,thethoughtexperiment,Avicennatellsus,isonlyawayofarousing(tanbīh)
inussomeconsiderationofwhatanimmaterialexistencemightbelike(Avicenna,
Deanima,1.1,15).
Interestingly,Avicennaalsousesacloseetymologicalcousinoftanbīh,
namely,tanabbuh,again“arousing,”inassociationwiththeaimofinduction
(istiqrāʾ)(Avicenna,Burhān,3.5,158).Induction,Avicennainformsus,cannot
establishsomeuniversallytrueclaim,butatbestcanonlyshowthatsomethingis
probable(Avicenna,Burhān,1.9,48).Whilethelinkisadmittedlytenuousitdoes
suggestthatAvicennamayhaveviewedthoughtexperimentsasatleastonparwith
inductioninscientificpractice.
Avicenna,however,doesnotusefictionalthoughtexperimentssolelyas
incitements,whichplaynosubstantiveroleindemonstrations.Insomecases,they
formanintegralpartofademonstrationasinindirectproofs.Infact,Avicenna
reliesonthoughtexperimentsandtheuseoftheestimativefacultyinjustthisway
scoresoftimesthroughouthisPhysics.ExamplesincludePhysics,2.1when
discussingself-motion(discussedindepthinKukkonen2014);numerously
throughoutPhysics,2.7–9and4.11,whendiscussingplace,voidandspace
(McGinnis2007aandLammer2016);thoughtexperimentsalsofrequentlyappear
inhiscriticismoftheinfinitelysmall,i.e.,atomismatPhysics,3.4–5(Lettinck1999
11
andMcGinnis2015)andtheinfinitelylargeatPhysics,3.7–9(McGinnis2010).These
arejusttomentionsomeofthemoreprominentappearancesofthought
experimentswithintheworksofAvicenna.Letmeconsiderbrieflysomeof
Avicenna’scommentsconcerningthevoidandhowonethoughtexperiment
featuresprominentlyinhisrefutationofit.
Avicennaintroducesthenotionofavoid(khalāʾ),byclaimingthatits
proponentsappealedtoacertainthoughtexperimenttomotivatetheirposition
(Avicenna,Physics,2.6[5]).Inthethoughtexperiment,theproponentsofthevoid
considersomecontainedbody,whetherthewaterinajugorwhatliesbetweenthe
moon’sorbitaroundtheearth.Herethecontainedbodyexistswithincertainlimits
ofthecontainingbody.They,then,throughanactoftheestimativefaculty,Avicenna
continues,imaginethatthecontainedbodyiseliminated;however,theelimination
ofthecontainedbodydoesnoteliminatetheintervalordimension(buʿd)between
thelimitsofthecontainingbody.Whatiseliminatedandwhatisnoteliminated,
however,aredistinctthings.Thus,thethoughtexperimentconcludes,theintervalor
dimensionisdistinctfromthebodyexistinginit,albeit,thatintervalissomething
existingtogetherwiththebodywhenthebodyexistsinit.
Avicenna’scriticismofthisargumentispreciselytoappealtothelimitsofthe
estimativefaculty’sabilities(Avicenna,Physics,2.9[11]).Tobegin,Avicennahappily
endorsesthegeneralmethodofanalysis(taḥlīl)thatthethoughtexperiment
employs:oneusestheestimativefacultytoisolatesomeformalfeaturewithina
bodyforcloserscrutiny.Infact,Avicennamaintainsthatitisjustthismethodthat
allowsoneconceptuallytodistinguishtheformofabodyfromitsmatter.The
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probleminthepresentcasecomesfromthinkingthatwhatisseparableinthought
mustalsobeseparableinreality.Heclarifiesbyappealingtotheform-mattercase:
Wereoneabletoremoveallformsfromsomematter,thematter,Avicenna
observes,wouldsimplyceasetoexist,fortheformistheprincipleofactualization.
Asforthecaseoftheimaginedvoidinterval,hewrites:
Letusgrantthatthisintervalisassumedintheestimativefaculty,whena
certainbodyorbodiesareeliminated.Howdoesoneknowthatthisactofthe
estimativefacultyisnotfalse[whenappliedtosomethingexistingseparate
fromtheestimativefaculty],suchthatwhatfollowsuponitisabsurd,and
whetherthisassumptionis,infact,evenpossible,suchthatwhatfollows
uponitisnecessary?(Avicenna,Physics,2.9[11])
Avicenna’scomplaintistwofold.First,ifthethoughtexperimentistoshowthe
extra-mentalexistenceofavoidinterval,onemustshowthataseparatevoid
intervalcanexistseparatefromanactoftheestimativefaculty.Inotherwords,one
mustdemonstratethattheseparateexistenceofavoiddoesnotleadtosome
absurdity,asintheform-mattercase,wheretheactualeliminationofformwould
entailtheactualizationofmatterwithoutitshavinganyprincipleofactualization,
i.e.,anyform.Second,evenassumingthatonecanshowthattheseparateexistence
ofavoidispossible,thethoughtexperimenthasnotshownthatavoid’sexistenceis
necessary.Ahallmarkofscientificknowledge,however,whichgoesbackatleastas
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farasPlatoandAristotle(cf.Plato,Theatetus,152C3andAristotle,Posterior
Analytics,A.2,71b9–12),isthatscientificknowledge(Gk.,epistēmē,Ar.ʿilm)is
necessaryandexplanatoryofwhatis.Avicennaacceptsthesecriteriaforknowledge.
Thushecomplainsthatthethoughtexperimentalonehasfailedtomeetoneofthe
conditionsforknowledge;whatisadditionallyneededtoshowthatavoid’s
existenceisnecessaryisademonstration.
NoneofthisistosaythatathoughtexperimentforAvicennacannotbean
integralpartofademonstration.Hisownrefutationofthevoidprovidesonewith
justsuchanexample.AtPhysics,2.8,Avicennaaimstoshowthattheexistenceofa
voidwouldmakemotionimpossible.Heidentifiesthreegeneralsortsofmotion:
naturalcircularmotion(suchasthatoftheheavens),naturalrectilinearmotion
(suchasthatoftheelements,earth,water,airandfire)andfinallyforcedmotion
(suchasaprojectilelikeanarroworathrownball).Thoughtexperimentsinthe
formofindirectproofsfortheimpossibilityofavoidappearinAvicenna’streatment
ofallthreeclassesofmotion.Ishallconsiderjustone:hisrefutationofthe
possibilityofforcedmotioninavoid(Avicenna,Physics,2.8[18]).
Theargumentbeginsbyimaginingalongwiththeproponentsofthevoidthat
aninfinitevoidexistsinwhichobjectsmove.Nowinthecaseofforcedmotion,for
examplemyshootinganarrow,I,bymeansofthebow,impartacertainmotive
powertothearrow.Giventhisscenario,eitherthearrowwillcontinueinitsmotion
unabatedinfinitelyoritwillcometoastop.Thearrowcannotcontinueoninfinitely,
3AdmittedlyPlatoisspeakingaboutperceptionhere,butthesuggestionisthatperceptionjustisknowledgebecauseithasthehallmarksofknowledge:itisaboutwhatisandisinfallible(apseudēs).
14
Avicennabelieves,forafiniteagent,andIamfinite,canonlyeverproduceafinite
effect,butshouldthearrowcontinuemovingwithouteverstopping,Iwouldhave
producedaninfiniteeffect.Ifthearrowceasestomove,thentheprivationor
absence(ʿadam)ofmotionmustbelongtothearroweitheressentiallyorowingto
someexternalcause.Ifnotmoving,thatis,theabsenceofmotion,belongedtothe
arrowessentially,thenitsmotionwouldbeimpossiblefromthestart,foritsessence
wouldprecludeitsmoving.Asforanexternalthing’sbringingthearrowtorest,we
havebeenaskedtoimagineavoid,andsosomethingliterallydevoidofanycauses
thatmightarrestthearrow’smotion.Ofcourse,Avicennadevelopseachofthese
momentsintheargumentingreaterdetail,butalmosteverymomenthasone
imagininghowtheprojectilewouldmoveorcometorestinavoid.
Inthiscaseandtheotherswherethoughtsexperimentsareintegraltothe
demonstrationAvicennaisnotrestrictedtolimitinghisconclusiontoamere
possibilityexistingintheestimativefaculty.Thatisbecausetheseargumentsare
intendedpreciselytoshowthattheseparateexistenceofthesubjectofthethought
experimentisimpossible.Thus,iftheinitialsuppositionplusasetofauxiliary
premises,allofwhicharetakentobetrueorevennecessary,leadtoanabsurdityor
impossibility,theinitialsuppositionmustbejettison.Noneofthisisnewto
Avicenna.Still,itdoessuggestthatAvicennawasprincipledwithrespecttohisuse
offictionalthoughtexperiments:eithertheymustbeaccompaniedbyan
independentdemonstrationortheyareconcededbecauseone’sopponentactually
acceptsthemastruedepictionsofreality.
15
4.IdealizedThoughtExperimentsintheMedievalIslamicWorld
Whatdistinguishesidealizedthoughtsexperimentsfromfictionalthought
experimentsisthattheformeratleastgivetheappearancethattheyarephysically
possibleandsocouldactuallyberealizedwithoutnecessarilyappealingtothe
actionofanall-powerfulagent.Iconsidertwobroadclassesofidealizedthought
experimentsusedinthemedievalIslamicworld:mechanicalthoughtexperiments
andnon-mechanicalones.Mechanicalthoughtexperimentsappealtosome
ingenuousmachineorapparatusandattheveryleastgivetheimpressionthatone
couldactuallycarryouttheexperimentorbuildtheapparatus.Beforeturningto
thesemechanicalthoughtexperiments,letmebeginwithaclassicexampleofanon-
mechanicalidealizedthoughtexperiment.
InhisIncoherenceofthePhilosophers,al-Ghazālī(1058–1111)challengesthe
philosophers’insistencethataprincipleofsufficientreasonmustgovernallactions.
Hedeniesthattheprinciplenecessarilyapplieswhenitcomestothechoicesof
volitionalagents.Morespecificallyal-Ghazālīwantstoshowthatevenifpresented
withtwocompletelyindiscernibleoptions,Godandevenhumanscan,unlike
Buridan’sass,chooseoneovertheother.Hisargumentforthisconclusionrelies
solelyonthefollowingidealizedthoughtexperiment:
Letussuppose(nafriḍu)twoindiscernibledatesimmediatelybefore
someonewholooksonthemhungrily,butisincapableoftakingboth.Hewill
takeoneofthemnecessarilythroughanattributewhosecharacteristo
16
specifyonethingfromitslike.Everythingyoumentionedconcerning
specificationsofsuperiority,proximityorfacilityofaccess,wedetermine,by
supposition(ʿaláfarḍ),tobeabsent,butthepossibilityoftakingremains.
Youhavetwooptions:either(1)tosaythattheindiscernibilityinrelationto
hisdesiresiswhollyinconceivable,whichisfatuousgiventhatthe
supposition[ofthedate’sindiscernibility]ispossible,or(2)tosaythatwhen
theindiscernibilityissupposed,thehungrilylongingmanwouldalways
remainundecided,staringatthetwo[dates],butnottakingeitherofthem
simplybywilling,butchoosingtostandalooffromthedesire,whichisalso
absurd,whosefalsityisknownnecessarily.(al-Ghazālī,Incoherence,Disc.1
[46])
Theargumentisstraightforward.Weareaskedtoimagineanidealizedsituation
whereeveryconceivablefactorforpreferringonedesiredoptionoveranotherhas
beeneliminated.Al-Ghazālītakesitaspatentlypossiblethattheimaginedscenario
couldexistintheworldandnotmerelyintheestimativefaculty.Ifthesituationis
possible,thenitiscertainlypossiblethatthehungrymanwillchooseonepieceof
fruitoveranotherwithoutanyreasonweighinginforhispreferenceforthat
particularpiece.Indeed,al-Ghazālīthinksthatchoosinginthissituationisnot
merelypossiblebutnecessary.Hethusconcludesthateveninhumanstheremust
besomepsychologicalfacultythatchoosesbetweenindiscerniblethings,called
“will”or“volition”(irāda).
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IknowofnophilosopherworkingwithinanAvicennanpsychological
frameworkwhoaddressesthisthoughtexperiment.Presumably,ifconfrontedwith
it,Avicennawouldhaverequiredsomeproofthattheimaginedscenariocouldexist
intheworldandnotmerelyintheestimativefaculty.
PerhapsamoreinterestingresponsecomesfromtheAndalusianPeripatetic,
IbnRushd(1126–1198),thatis,Averroes.AlthoughAverroes’commentssaylittle
aboutthenatureofidealizedthoughtexperimentsasaclassofarguments,theyare
informativeaboutthepresentexample(Averroes,TheIncoherenceofthe
Incoherence,Disc.1,[39–41]).Averroescomplainsthatal-Ghazālī’sthought
experimentdoesnotsetoutoneuniquesetofpreferences,forexample,topreferto
eatdate1ordate2.Instead,observesAverroes,therearetwodistinctsetsof
preferences:(1)toeatornottoeatand(2)toeatdate1ordate2.Ofcoursewith
respecttoset(1),thehungrymanhaseveryreasontoprefertoeatovernoteating,
andsoindeedwillstoeatonthebasisofthatreason.Thatactionisachieved
regardlessofwhetherheeatsdate1ordate2.Asforcase(2),ifthemanwere
subsequentlyaskedwhyhepreferred,forexample,date1overdate2,hewouldsay
thathedidnotprefertheonedateovertheother;hesimplypreferredtoeatrather
thannottoeat.Thus,whilethereisnoreasonforpreferringonedateoveranother,
neitheristhereanypreferenceforonedateoveranotherthatneedsareason.Again,
however,thereseemslittletogleanfromAverroes’discussionhereaboutthenature
ofthoughtexperimentsoridealization(althoughseeKnuuttilaandKukkonen2011,
esp.§2).
18
Turningnowtothemechanicalvarietyofidealizedthoughtexperiments,for
obviousreasonstheywerealmostexclusivelyappliedtoissuesandproblemsinthe
naturalsciences.Thatisbecausethisclassinvolvesdescribingamachineor
apparatusthatcan,atleastinprinciple,beconstructedandassuchmustbe
constrainedbythelawsofphysicsandprinciplesofmechanics.Onesuchphysical
issue,inwhichtherewasaproliferationofmechanicalthoughtexperiments,was
theproblemofthequiesmedia,thatis,medialrest.
Theissueatstakeiswhetherabodythatundergoescontrarychangesmust
cometosomerestbetweenonechangeandthenthecontrarychange.Forexample,
mustaballthrownupwardcometoaslightrest,beiteversoshort,beforeitmoves
downwardorcantheballchangefrommovingupwardtomovingdownward
Instantaneously?AristotleinhisPhysics,8.8,hadarguedforamedialrest.His
generallyargumentassumedsomethinglikethefollowingform.Letabodymove
fromAtoC.AteverymomentinitsmotionfromAtoC,thebodyisinaprocessof
arrivingatC,whereasateverymomentinitsmotionfromCbacktoAthebodyisin
aprocessofarrivingatA.AisnotC,andsoduringthebody’smotionbacktoA,itis
notinaprocessofarrivingatC.Nowto-be-in-a-process-of-arriving-at-Candnot-to-
be-in-a-process-of-arriving-at-Carecontradictorypredicates,andnothingcan
simultaneouslyhavecontradictorypredicates.Hence,reasonsAristotle,theremust
besomeinstantatwhichthebodyarrivesatCandsomeotherinstantatwhichit
departsC.Finally,sincetimeiscontinuousandbetweenanytwopoints(orinthis
caseinstants,i.e.,temporalpoints)onacontinuumthereissomemagnitude,there
19
mustbesometemporalmagnitude,andsosometime,betweentheinstantofthe
body’sarrivingatCandofitsdepartingfromCwhenitisatrestatC.
Aristotle’swordonthissubjectwasfarfromthelast.Indeed,theissuewas
stillverymuchaliveinAvicenna’stime(seeRashed1999,esp.§2,Morrison2005,
58–9&91–2andLangermann2008).Infact,theissuehadgeneratedsomuch
unrestthatAvicennadedicatedanentirechapterofhisPhysics,4.8,totheproblem.
Avicennahimselfevenconfessesthathedidnotfindtheargumentsoneitherside
particularlyimpressive(Avicenna,Physics,4.8[9]).Theissueisparticularlypressing
forAvicennasinceheprovidedananalysisofmotionthatallowedformotionatan
instantinsuchawaytoavoidAristotle’sconclusion(seeHasnawi2001,McGinnis
2006andAhmed2016).Asfortheargumentsproandcon,Avicennanotesthatthe
mainpremiseinargumentsfortherebeingarestbetweencontrarymotions
involvesidentifyingsomepurportedimpossibilityinthesituation,likethe
contradictionthatAristotlementionedintheabovecase.Hefurthernotesthatthe
counterargumentssimplyneedtoshowthattheinstantaneouschangefromone
typeofmotiontoitscontraryisnotimpossible.ThosewhoopposedAristotle’s
conclusionappealedtothislastpoint,andaneasyenoughwaytoshowthat
possibilityissimplytodescribeamachinethatproducesjustsuchamotion.
Avicennahimselfmentionsonesuchcontraptionproposedbythedetractors
ofamedialrest(Avicenna,Physics,4.8[4]).Wearetoimagineaspheremounted
uponawheelandthewheelmakesacontinuousrotation.Nextimagineatwo-
dimensionalplaneabovetheapparatusthatissituatedsuchthat,whenthesphereis
atitsapexduringthewheel’srotation,thesphereencountersthatplaneatsome
20
singlepoint,C.Sincethewheelthatiscarryingthesphereismovingcontinuously,
thespherewilltouchCforonlyaninstant.Thus,duringthesphere’sascentitwill
havebeeninaprocessofarrivingatC,whileduringitsdescentitwillbeinprocess
ofdepartingfrom,i.e.,notarrivingat,C,justasAristotledescribes,andyetcontrary
toAristotle,thespherewillbeatCforonlyaninstant.Whiletheexampletakes
advantageofcircularmotion,itdoessuggestthatabodycanactuallybeatapoint
foraninstantandatthatinstantchangefromonesortofmotiontoitscontrary.
Asalreadynoted,Avicennawasnotimpressedwithanyoftheavailable
argumentsconcerningthisissue,proorcon.4Hisobjectiontothepresentone
involvesadigressionaboutthenatureofmechanicalthoughtexperimentsmore
generally(Avicenna,Physics,4.8[12]).Hisconcernsaremuchlikethoseregistered
aboutfictionalthoughtexperiments.Thedifferenceisthat,whileinthecaseof
fictionalthoughtexperimentsonemustprovideademonstrationthatwhatthe
thoughtexperimentdescribescanexistseparatefromtheestimativefaculty,ina
mechanicalthoughtexperimentsonemustfirstaskiftheproposedmachinecanin
factactuallywork.Canitbeconstructedinprinciple?Ifitcannot,thenthethought
experimentmustbetreatedasifitisafictionalone.
Todaywemightthinkthattheproposedthoughtexperimentimmediately
failsthecan-it-worktestsinceitappealstoperfectsspheresandplanes,i.e.,
mathematicallyidealizedones,ratherthantophysicalones.Infact,Avicenna
mentionedthatsomecomplainedaboutthisthoughtexperimentsinjustthisway.
4WhileAvicennaultimatelywillagreewithAristotlethattheremustbearestbetweencontrarymotions,hisownuniqueargumentforthisthesisappealstotheforcesproducingthemotionsratherthanthesortsofmotioninvolved.
21
Avicennadismissestheobjectionasinadequate,sinceinhiscosmologythere
actuallywereperfectlyinstantiatedspheres,namely,thecelestialspheresthatcarry
theplanetsalongtheirorbits.Consequently,atleastpartoftheimaginedapparatus
can,byAvicenna’slights,bephysicallyinstantiated.
Avicenna’scomplaintcomeswhenconsideringwhethertheserotating
spherescancomeincontactwithaflatsurfaceatasinglepoint.Theperfectspheres
thatAvicennapermitsareembeddedwithinoneanother.Thus,whileaspheremay
beincontactwiththesurfaceofanothersphere,onAvicenna’sview,itwouldnot
contactitatasinglepointbutinitsentirety,eithercontainingorbeingcontained.
Heinfactarguesthatonecandemonstratethephysicalimpossibilityofasphere’s
actuallytouchingaflatplaneatasinglepoint,evenifonegrantsthephysical
existenceofbothaperfectsphereandatwo-dimensionalsurface.Hereasonsthusly:
betweentheflatsurfaceandthespheretheremustbeavoidornot.AtPhysics,2.8,
Avicennaspiltmuchinktoshowthatavoidisnotonlyphysicallyimpossiblebut
alsoconceptuallyimpossible(McGinnis2007b,esp.§IVandLammer2016,§5.3).
Thus,iftheproposedapparatusentailsavoid,thenitentailsanimpossibility,andso
mustitselfbeavacuousproductoftheimagination.
Ifthereisnovoid,Avicenna’sargumentcontinues,thentheremustbea
plenumwhosesurfacecontactstheflattwo-dimensionalplaneandtheconvex
surfaceofthesphere.Now,accordingtothecontinuoustheoryofphysicalbodies,
whichAvicennaadopts,pointshavenodeterminateexistenceinacontinuous
surface,saveasendpointsoflines.Consequently,Avicennagoeson,pointsexistin
thecontinuumonlyifthereisaphysicalseparationofthecontinuoussurface,in
22
whichcasethepointexistsasanendpoint,otherwiseitexistsmerelyasaproductof
theestimativefaculty’spositingthepoint.Thus,Avicennacontinues,itisimpossible
thatthesinglepointofthesphereshouldhavesomeseparate,determinateposition
inthesurfaceoftheplenumthattouchestheflattwo-dimensionalplanegiventhe
verynatureofcontinua.
Heconcludeshiscritiqueofthisthoughtexperimentthus:
This[argument]makesthelawsofnaturedependentuponcertain
mathematicalabstractionsoftheestimativefaculty,whichisnotright.Infact,
beyondgoingoutsidethediscipline[ofphysics],that[argument]doesn’t
evenentailwhat[they]wantedittoprove,butonlyrequiresthatthe
continuityofthetwodesignatedmotionsbeintheestimativefaculty.We,
however,don’tdenythatthatcontinuityisintheestimativefaculty.Wedeny
[thecontinuity]onlyofthenaturalthingsthatdeviatefromtheabstractions
oftheestimativefaculty.(Avicenna,Physics,4.8[12])
Asacurioushistoricaladdendum,thepost-Avicennanpolymath,Quṭbal-Dīn
Shīrāzī(1236–1311),showedhowamodelusedinastronomycouldproducea
continuousmotionbetweenabodythatascendsandthendescend,whichisperhaps
immunetoAvicenna’scriticism(Morrison2005,58–9&91).Shīrāzītookadvantage
ofamathematicaldevise—theeponymousṬūsīcouple—thatNaṣīral-Dīnal-Ṭūsī
(1201–74)hadconstructedtobringaboutabettermatchbetweenastronomical
observationsandthepredictionsofthegeocentricmodeloftheuniverseadoptedby
23
ancientandmedievalastronomers.TheṬūsīcoupleassumestwocontinuousand
uniformlyrotatingcircles(butthedevicecouldalsobeconstructedusingspheres).
Onecircleisinsidetheotherwiththecontainedcirclehavinghalfofthediameterof
thecontainingcircleandrotatingtwiceasfastasandintheoppositedirectionasthe
containingcircle.Theoveralleffect,Ṭūsīobserved,isthatacertainpointonthe
circumferenceofthesmallercircleoscillatesupanddownthediameterofthelarger
circle.Thisoscillating“point”wassubsequentlyidentifiedwithsomeplanet.5
Shīrāzī’scontributiontothedebateaboutmedialrestwastonotethatsincesolely
continuousrotationsproducetheoscillation,thepoint/planetwillcometoone
endpointofthediameterandthenwithoutrest(fortherotationsdonotstop)
instantaneouslymovebacktowardtheotherendpoint.Consequently,totheextent
thatonebelievedthattheastronomicalmodelusedinPtolemaicsystemsdescribed
theactualworkingsoftheheavens,theṬūsīcouplewouldpassAvicenna’scan-it-
worktest.
Letmeconcludewithonefinalsetofpossiblemechanicalidealizedthought
experiments,nowdrawnfromthegreatmedievalMuslimoptician,Ibnal-Haytham.6
(Isay,“possible”becauseforsometheexamplesthatIgiveareseenasinstancesof
actualexperimentsratherthanthoughtexperiments;Iletthereaderdecide.)
ThroughouthisBookofOptics(Kitābal-Munāẓir,Lt.Deaspectibus)Ibnal-Haytham
takeswhatbyallappearancesisastaunchempiricalapproachtothestudyofoptics,
suggestingnumerousexperimentsandapparatustoverifyempiricallyvarious
5AgraphicrepresentationcanbefoundonWikipediaunder“TusiCouple”:https://en.wikipedia.org/wiki/Tusi_couple.6ThepositionIpresentheredrawsheavilyuponA.MarkSmith2015:ch.5.
24
principlesusedinoptics.Innumerouscases,hesuggestsasetofexperimentsthat
requiretheconstructionofhighlypreciseapparatuses,whichhedescribeswith
meticulouscare,indeedsuchcarethattheyappeartobemoreidealizationsthan
devicesactuallyusedbyhim.Unfortunately,theexactingdetailsofhisdescriptions
andthelengthitwouldtaketodescribethemprecludepresentingevenoneofthem
hereinfulldetail,althoughoneexamplemighthelp
makemypoint.
Whenconsideringrefraction,Ibnal-Haytham
describesanapparatusfortestingtherefractive
propertiesofdifferentmedia,whichrequiresasone
ofitsparts,arelativelylargequartersphereofglass
(like,forexample,ABCDinthediagram).Raysoflight
areallowedtopassthroughthequarterspherebeforepassingthroughadifferent
mediumlikeairorwater.Thetechnologicalstateofglassworking,particularlyata
timebeforemachine-producedglassware,makesitdifficulttoassumethatany
quartersphereproducedinIbnal-Haytham’stimewouldhavebeenfreeofthe
variousflawsthattypifyhandmadeglassitems,suchasthetinybubblesorvarious
stretch,mold,shearorpontilmarks.Theseimperfections,however,wouldhave
distortedtheobservedresultsoftheexperiments.Similarly,forthedevicetogive
themathematicallyexactresults,whichIbnal-Haythamclaims,thetwoflatsurfaces
ofthequarterspherewouldneedtobeexactlyperpendicularandthecurved
surfaceperfectlyconvex,againfeaturesthatseemallbutimpossiblegiventhe
technologyofthetime.
A
B
CD
25
Ibnal-Haythammakessimilarexactingdemandsonthespecificationsfor
anotherapparatus,nowusedinvalidatingtheequal-angleslawfoundindiscussions
ofreflection.Notedhistorianofscience,A.MarkSmith,hasthistosayaboutthe
levelofprecisionrequiredofthatapparatusinordertogetthepurportedresults:
Indeed,given[theapparatus’]obviousunfeasibilityasactuallydescribed—
withallplanesperfectlyalignedandallmeasurementsperfectly
reproduced—thetestappearstohavebeenanelaboratethought
experimentsdesignedtoconfirmwhat[Ibnal-Haytham]alreadytookfor
granted,thatis,thatlightreflectsatequalangles.(Smith2015,199)
ItisnotmyintentiontodiminishthesignificanceofIbnal-Haytham’scontribution
tooptics—itisimpressiveindeed—rather,Imerelywanttosuggestthatsomeofhis
experimentsmightbestbeclassifiedasinstancesofwhatIhavebeencalling
idealizedthoughtexperiments.
4.Conclusion
TherecanbelittledoubtthatthinkersinthemedievalIslamicworld
appreciatedtheroleandsignificanceofthoughtsexperimentsforphilosophyand
thesciences.Indeedtheprevalenceofthoughtsexperimentsintheseareasseemsto
haveleadAvicennatoexplorethepsychologybehindthemandpresentrulesfor
determiningacceptableandunacceptableuseofpremisesrelyingonthem.
26
Helaudedandevenemployedthoughtexperimentshimselfwhenusedas
intuitionpumpsprimarilytoarouseinusabetterunderstandingofsome
independentlyprovenpoint.Whenthoughtexperimentswereintegralpartsofa
proof,however,hewasmorehesitant.Ifthoughtexperimentswereusedinindirect
proofs,theyneededtobepartofaconditionalpremise,ideallyfunctioningasthe
consequenceofsomehypothesiswhoseverypossibilitywasbeingquestioned.In
anyotheruse,Avicennastressesthatonemustprovethatthesituationimaginedin
thethoughtexperimentscanactuallyoccurintherealworld,andsodoesnothave
itsexistencemerelyasaproductofthemind,orparticularly,oftheestimative
faculty.Inotherwords,whenthoughtexperimentsweretoplaysomeintegralrole
inaproof,Avicennarequiresthattheassumedscenariobeexecutableatleastin
principle.
Ascenariocouldbeshowntobeexecutableinprinciple,atleastinsome
cases,ifitreliesonamechanicalapparatusthatdidnotviolateanyphysical
principles.Indeedthereappearstohavebeenaproliferationofidealized
mechanicalthoughtsexperimentswithinnaturalphilosophy,whetherphysics
proper,optics,astronomyorthelike.Suchaproliferationatleastsuggeststhat
physicswithinthemedievalIslamicworldwasbeginningtoshowthefirst
tendenciesofanexperimentalapproachtothesciences,althoughthistendencyis
betterdescribedintermsof“methodologicalexperience”(seeMcGinnis2003and
Janssens2004).Whateverthecase,theuseofthoughtexperimentswouldhavebeen
partofanempiricismthatwenthandinhandwithamarkedrationalistleaning.
27
Asforevidenceofthislastpoint,wehaveseenAvicenna’sdemandthat
thoughtexperimentsreallyneedtobeaccompaniedbyademonstration(burhān).A
demonstrationinthiscontextwouldhavemeantalogicallyvalidsyllogism
proceedingfromnecessaryfirstprinciples,wherefirstprinciplesareproductsofthe
intellect(ʿaql)nottheestimativefaculty.Thissametendencyalsoseemspresentin
theexperimentsofIbnal-Haytham.Thepunctiliousprecisionwithwhichhe
describedtheapparatusthathisexperimentsemployedallbutnecessitatesthatthe
demandsofanidealizedmathematicaldemonstrationsdirectedhisdetailed
instructions.Finally,themedievalJewishphilosopher,Abūl-Barakāt(1080–1165),
aftercatalogingvariousargumentsagainstaquiesmedia,notesthatthosewhofavor
therebeingamedialrestwouldonlybesatisfiedbyademonstrationproceeding
fromtheintellect(Abūl-Barakāt,al-Muʿtabar,2.14,97).Byimplicationmechanical
arguments,whetherofthethoughtexperimentvarietyorotherwise,wouldhave
takenabackseattoaproperintellectualdemonstration.
Toconclude,thoughtexperimentsinthemedievalIslamicmilieuseemedto
functionasahalfwayhousebetweenempiricismandrationalism,allowingthe
idealizationthatrationalismdemandswhilealsoappealingtosensibleintuitions
favoredbyanempiricistapproachtothesciences.
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