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Sincetherewerethreekindsofsubstance，twoofthemphysical
andoneunmovable，regardingthelatterwemustassertthatitis
necessarythatthereshouldbeaneternalunmovablesubstance。For
substancesarethefirstofexistingthings，andiftheyareall
destructible，allthingsaredestructible。Butitisimpossiblethat
movementshouldeitherhavecomeintobeingorceasetobe(forit
mustalwayshaveexisted)，orthattimeshould。Fortherecouldnotbe
abeforeandanafteriftimedidnotexist。Movementalsois
continuous，then，inthesenseinwhichtimeis；fortimeiseither
thesamethingasmovementoranattributeofmovement。Andthereis
nocontinuousmovementexceptmovementinplace，andofthisonlythat
whichiscirculariscontinuous。
Butifthereissomethingwhichiscapableofmovingthingsor
actingonthem，butisnotactuallydoingso，therewillnot
necessarilybemovement；forthatwhichhasapotencyneednot
exerciseit。Nothing，then，isgainedevenifwesupposeeternal
substances，asthebelieversintheFormsdo，unlessthereistobein
themsomeprinciplewhichcancausechange；nay，eventhisisnot
enough，norisanothersubstancebesidestheFormsenough；forifit
isnottoact，therewillbenomovement。Furtherevenifitacts，
thiswillnotbeenough，ifitsessenceispotency；fortherewillnot
beeternalmovement，sincethatwhichispotentiallymaypossibly
notbe。Theremust，then，besuchaprinciple，whoseveryessenceis
actuality。Further，then，thesesubstancesmustbewithoutmatter；for
theymustbeeternal，ifanythingiseternal。Thereforetheymustbe
actuality。
Yetthereisadifficulty；foritisthoughtthateverything
thatactsisabletoact，butthatnoteverythingthatisableto
actacts，sothatthepotencyisprior。Butifthisisso，nothing
thatisneedbe；foritispossibleforallthingstobecapableof
existingbutnotyettoexist。
Yetifwefollowthetheologianswhogeneratetheworldfrom
night，orthenaturalphilosopherswhosaythat’allthingswere
together’，thesameimpossibleresultensues。Forhowwilltherebe
movement，ifthereisnoactuallyexistingcause？Woodwillsurelynot
moveitself-thecarpenter’sartmustactonit；norwillthemenstrual
bloodnortheearthsetthemselvesinmotion，buttheseedsmustact
ontheearthandthesemenonthemenstrualblood。
Thisiswhysomesupposeeternalactuality-e。g。Leucippusand
Plato；fortheysaythereisalwaysmovement。Butwhyandwhatthis
movementistheydosay，nor，iftheworldmovesinthiswayor
that，dotheytellusthecauseofitsdoingso。Nownothingis
movedatrandom，buttheremustalwaysbesomethingpresenttomove
it；e。g。asamatteroffactathingmovesinonewaybynature，and
inanotherbyforceorthroughtheinfluenceofreasonorsomething
else。(Further，whatsortofmovementisprimary？Thismakesavast
difference。)ButagainforPlato，atleast，itisnotpermissibleto
nameherethatwhichhesometimessupposestobethesourceof
movement-thatwhichmovesitself；forthesoulislater，andcoeval
withtheheavens，accordingtohisaccount。Tosupposepotencyprior
toactuality，then，isinasenseright，andinasensenot；andwe
havespecifiedthesesenses。Thatactualityisprioristestifiedby
Anaxagoras(forhis’reason’isactuality)andbyEmpedoclesinhis
doctrineofloveandstrife，andbythosewhosaythatthereisalways
movement，e。g。Leucippus。Thereforechaosornightdidnotexistfor
aninfinitetime，butthesamethingshavealwaysexisted(either
passingthroughacycleofchangesorobeyingsomeotherlaw)，since
actualityispriortopotency。If，then，thereisaconstantcycle，
somethingmustalwaysremain，actinginthesameway。Andifthere
istobegenerationanddestruction，theremustbesomethingelse
whichisalwaysactingindifferentways。Thismust，then，actin
onewayinvirtueofitself，andinanotherinvirtueofsomething
else-eitherofathirdagent，therefore，orofthefirst。Nowit
mustbeinvirtueofthefirst。Forotherwisethisagaincausesthe
motionbothofthesecondagentandofthethird。Thereforeitis
bettertosay’thefirst’。Foritwasthecauseofeternaluniformity；
andsomethingelseisthecauseofvariety，andevidentlyboth
togetherarethecauseofeternalvariety。This，acc
