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TheoriesofLocalMotionbeforetheOxfordCalculators
S=c(W-R)/W,
45
uwherecisaconstant,Sthespeed,Wtheweightofthebody,andRthe
resistanceofthemedium.Inavoid(whereR=0),S=c,whateverthe
valueofW,yieldingtheabove-mentionedresultthatanytwobodiesfall
throughavoidatequalspeed,whatevertheirweight.”17
AvicennarejectsPhiloponus’interpretationofAristotlebecauseof
aprecisereasonexplainedindetailbyRashed,who,afteradetailed
studyofAvicenna’sPhysicsoftheShifā’(Bk.II,Ch.I),concludesasfol-
lows:
Avicennastressesthatwecanmeantwothingswhenwespeakof
umotion”:motionasatrajectory,whichpertainstoourimagina-
tivefacultyandisconceivedofonlyaslinkingastartingpointto
anend;andmotionasanintermediatestate,whichmustbeattrib-
utedtoeachmomentofthetrajectory.Motioninthesecondsense
characterizesasaninfnitesimalmoment,andnothingelse.(…)
Eachsubstancespatiallyorqualitativelyremovedfromitsnatural
state(e.g.,astonethrownupawayfromitsnaturalresistingplace)
returnstoit,passingthroughallintermediarystates.Eachofthese
intermediarystates,becauseitisnottheendpointoftheprocess,
produceanewmayl,whichaddsitselftotheimpulsionproduced
bytheothers.Everymomentisthuscharacterizedbyitsownki-
neticintensity.(ł)Avicennathusseemstostandthecrossroadsof
twotraditions.Withthemathematicians,herecognizesthatevery
oneoftheinfnitepointsonaspecialintervalAB,withoutper-
hapsbeingperfectlyreal,ishowevernotionallyandqualitatively
distinctfromeveryotherpoint.Butwiththemutakallimūn,hesees
inadynamicofimpetustheeffcientprincipleofsuchdistinction.
Thus,startingfromaclassifcatoryprojectofthedifferenttypesof
impetus,Avicennaarrivesatacomplex-becausepartiallyuonto-
logical”-doctrineofinstantaneousmotion.Thiscombinationof
thekinematicsofthegeometersandthedynamicsofmutakallimūn
17SeeM.Rashed,Naturalphilosophy,pp.296-297.ItispossiblethatBradwardine
getsuseofthistheory,butheinterpretsitallwayaroundwhilestatingthats=
F-R/Rtoprovethatthespeedofmotionisnotcausedbytheproportionof
equality,whenF=R,thens=R-R/R=0/R=0.Thispeculiartheoryistobe
foundonlyinBradwardine’sTractatusdeproportionibus,pp.92-94.