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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.

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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:

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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

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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).

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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.

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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

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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).

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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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