WEhavetreatedofthatwhichisprimarilyandtowhichallthe
othercategoriesofbeingarereferred-i。e。ofsubstance。Foritisin
virtueoftheconceptofsubstancethattheothersalsoaresaidto
be-quantityandqualityandthelike；forallwillbefoundtoinvolve
theconceptofsubstance，aswesaidinthefirstpartofourwork。
Andsince’being’isinonewaydividedintoindividualthing，
quality，andquantity，andisinanotherwaydistinguishedin
respectofpotencyandcompletereality，andoffunction，letusnow
addadiscussionofpotencyandcompletereality。Andfirstletus
explainpotencyinthestrictestsense，whichis，however，notthe
mostusefulforourpresentpurpose。Forpotencyandactuality
extendbeyondthecasesthatinvolveareferencetomotion。Butwhen
wehavespokenofthisfirstkind，weshallinourdiscussionsof
actuality’explaintheotherkindsofpotencyaswell。
Wehavepointedoutelsewherethat’potency’andtheword’can’
haveseveralsenses。Ofthesewemayneglectallthepotenciesthat
aresocalledbyanequivocation。Forsomearecalledsobyanalogy，
asingeometrywesayonethingisorisnota’power’ofanotherby
virtueofthepresenceorabsenceofsomerelationbetweenthem。But
allpotenciesthatconformtothesametypeareoriginativesourcesof
somekind，andarecalledpotenciesinreferencetooneprimarykind
ofpotency，whichisanoriginativesourceofchangeinanother
thingorinthethingitselfquaother。Foronekindisapotencyof
beingactedon，i。e。theoriginativesource，intheverythingacted
on，ofitsbeingpassivelychangedbyanotherthingorbyitselfqua
other；andanotherkindisastateofinsusceptibilitytochangefor
theworseandtodestructionbyanotherthingorbythethingitself
quaotherbyvirtueofanoriginativesourceofchange。Inallthese
definitionsisimpliedtheformulaifpotencyintheprimary
sense-Andagaintheseso-calledpotenciesarepotencieseitherof
merelyactingorbeingactedon，orofactingorbeingactedon
well，sothatevenintheformulaeofthelattertheformulaeofthe
priorkindsofpotencyaresomehowimplied。
Obviously，then，inasensethepotencyofactingandofbeing
actedonisoneforathingmaybe’capable’eitherbecauseitcan
itselfbeactedonorbecausesomethingelsecanbeactedonbyit，
butinasensethepotenciesaredifferent。Fortheoneisinthe
thingactedon；itisbecauseitcontainsacertainoriginative
source，andbecauseeventhematterisanoriginativesource，thatthe
thingactedonisactedon，andonethingbyone，anotherby
another；forthatwhichisoilycanbeburnt，andthatwhichyieldsin
aparticularwaycanbecrushed；andsimilarlyinallothercases。But
theotherpotencyisintheagent，e。g。heatandtheartofbuilding
arepresent，oneinthatwhichcanproduceheatandtheotherinthe
manwhocanbuild。Andso，insofarasathingisanorganicunity，
itcannotbeactedonbyitself；foritisoneandnottwodifferent
things。And’impotence’and’impotent’standfortheprivationwhichis
contrarytopotencyofthissort，sothateverypotencybelongstothe
samesubjectandreferstothesameprocessasacorresponding
impotence。Privationhasseveralsenses；foritmeans1thatwhich
hasnotacertainqualityand2thatwhichmightnaturallyhaveit
buthasnotit，eitheraingeneralorbwhenitmightnaturally
haveit，andeitherainsomeparticularway，e。g。whenithasnot
itcompletely，orbwhenithasnotitatall。Andincertain
casesifthingswhichnaturallyhaveaqualityloseitbyviolence，we
saytheyhavesufferedprivation。
Sincesomesuchoriginativesourcesarepresentinsoulless
things，andothersinthingspossessedofsoul，andinsoul，andin
therationalpartofthesoul，clearlysomepotencieswill，be
non-rationalandsomewillbenon-rationalandsomewillbe
accompaniedbyarationalformula。Thisiswhyallarts，i。e。all
productiveformsofknowledge，arepotencies；theyareoriginative
sourcesofchangeinanotherthingorintheartisthimselfconsidered
asother。
Andeachofthosewhichareaccompaniedbyarationalformulais
alikecapableofcontraryeffects，butonenon-rationalpowerproduces
oneeffect；e。g。thehotiscapableonlyofheating，butthemedical
artcanproducebothdiseaseandhealth。Thereasonisthatscienceis
arationalformula，andthesamerationalformulaexplainsathingand
itsprivation，onlynotinthesameway；andinasenseitapplies
toboth，butinasenseitappliesrathertothepositivefact。
Thereforesuchsciencesmustdealwithcontraries，butwithonein
virtueoftheirownnatureandwiththeothernotinvirtueoftheir
nature；fortherationalformulaappliestooneobjectinvirtueof
thatobject’snature，andtotheother，inasense，accidentally。
Foritisbydenialandremovalthatitexhibitsthecontrary；forthe
contraryistheprimaryprivation，andthisistheremovalofthe
positiveterm。Nowsincecontrariesdonotoccurinthesamething，
butscienceisapotencywhichdependsonthepossessionofarational
formula，andthesoulpossessesanoriginativesourceofmovement；
therefore，whilethewholesomeproducesonlyhealthandthe
calorificonlyheatandthefrigorificonlycold，thescientificman
producesboththecontraryeffects。Fortherationalformulaisone
whichappliestoboth，thoughnotinthesameway，anditisinasoul
whichpossessesanoriginativesourceofmovement；sothatthesoul
willstartbothprocessesfromthesameoriginativesource，having
linkedthemupwiththesamething。Andsothethingswhosepotencyis
accordingtoarationalformulaactcontrariwisetothethingswhose
potencyisnon-rational；fortheproductsoftheformerareincluded
underoneoriginativesource，therationalformula。
Itisobviousalsothatthepotencyofmerelydoingathingor
havingitdonetooneisimpliedinthatofdoingitorhavingitdone
well，butthelatterisnotalwaysimpliedintheformer:forhewho
doesathingwellmustalsodoit，buthewhodoesitmerelyneed
notalsodoitwell。
Therearesomewhosay，astheMegaricschooldoes，thatathing
’can’actonlywhenitisacting，andwhenitisnotactingit
’cannot’act，e。g。thathewhoisnotbuildingcannotbuild，but
onlyhewhoisbuilding，whenheisbuilding；andsoinallother
cases。Itisnothardtoseetheabsurditiesthatattendthisview。
Foritisclearthatonthisviewamanwillnotbeabuilder
unlessheisbuildingfortobeabuilderistobeabletobuild，
andsowiththeotherarts。If，then，itisimpossibletohavesuch
artsifonehasnotatsometimelearntandacquiredthem，anditis
thenimpossiblenottohavethemifonehasnotsometimelostthem
eitherbyforgetfulnessorbysomeaccidentorbytime；foritcannot
bebythedestructionoftheobject，forthatlastsforever，aman
willnothavetheartwhenhehasceasedtouseit，andyethemay
immediatelybuildagain；howthenwillhehavegottheart？And
similarlywithregardtolifelessthings；nothingwillbeeither
coldorhotorsweetorperceptibleatallifpeoplearenot
perceivingit；sothattheupholdersofthisviewwillhaveto
maintainthedoctrineofProtagoras。But，indeed，nothingwilleven
haveperceptionifitisnotperceiving，i。e。exercisingits
perception。If，then，thatisblindwhichhasnotsightthoughit
wouldnaturallyhaveit，whenitwouldnaturallyhaveitandwhenit
stillexists，thesamepeoplewillbeblindmanytimesinthe
day-anddeaftoo。
Again，ifthatwhichisdeprivedofpotencyisincapable，that
whichisnothappeningwillbeincapableofhappening；buthewhosays
ofthatwhichisincapableofhappeningeitherthatitisorthatit
willbewillsaywhatisuntrue；forthisiswhatincapacitymeant。
Thereforetheseviewsdoawaywithbothmovementandbecoming。For
thatwhichstandswillalwaysstand，andthatwhichsitswillalways
sit，sinceifitissittingitwillnotgetup；forthatwhich，as
wearetold，cannotgetupwillbeincapableofgettingup。Butwe
cannotsaythis，sothatevidentlypotencyandactualityaredifferent
buttheseviewsmakepotencyandactualitythesame，andsoitis
nosmallthingtheyareseekingtoannihilate，sothatitispossible
thatathingmaybecapableofbeingandnothe，andcapableofnot
beingandyethe，andsimilarlywiththeotherkindsofpredicate；
itmaybecapableofwalkingandyetnotwalk，orcapableofnot
walkingandyetwalk。Andathingiscapableofdoingsomethingif
therewillbenothingimpossibleinitshavingtheactualityofthat
ofwhichitissaidtohavethecapacity。Imean，forinstance，ifa
thingiscapableofsittinganditisopentoittosit，therewillbe
nothingimpossibleinitsactuallysitting；andsimilarlyifitis
capableofbeingmovedormoving，orofstandingormakingtostand，
orofbeingorcomingtobe，orofnotbeingornotcomingtobe。
Theword’actuality’，whichweconnectwith’completereality’，
has，inthemain，beenextendedfrommovementstootherthings；for
actualityinthestrictsenseisthoughttobeidenticalwith
movement。Andsopeopledonotassignmovementtonon-existentthings，
thoughtheydoassignsomeotherpredicates。E。g。theysaythat
non-existentthingsareobjectsofthoughtanddesire，butnotthat
theyaremoved；andthisbecause，whileexhypothesitheydonot
actuallyexist，theywouldhavetoexistactuallyiftheywere
moved。Forofnon-existentthingssomeexistpotentially；butthey
donotexist，becausetheydonotexistincompletereality。
Ifwhatwehavedescribedisidenticalwiththecapableor
convertiblewithit，evidentlyitcannotbetruetosay’thisis
capableofbeingbutwillnotbe’，whichwouldimplythatthethings
incapableofbeingwouldonthisshowingvanish。Suppose，for
instance，thataman-onewhodidnottakeaccountofthatwhichis
incapableofbeing-weretosaythatthediagonalofthesquareis
capableofbeingmeasuredbutwillnotbemeasured，becauseathing
maywellbecapableofbeingorcomingtobe，andyetnotbeorbe
abouttobe。Butfromthepremissesthisnecessarilyfollows，that
ifweactuallysupposedthatwhichisnot，butiscapableofbeing，to
beortohavecometobe，therewillbenothingimpossibleinthis；
buttheresultwillbeimpossible，forthemeasuringofthediagonal
isimpossible。Forthefalseandtheimpossiblearenotthesame；that
youarestandingnowisfalse，butthatyoushouldbestandingis
notimpossible。
Atthesametimeitisclearthatif，whenAisreal，Bmustbe
real，then，whenAispossible，Balsomustbepossible。ForifBneed
notbepossible，thereisnothingtopreventitsnotbeingpossible。
NowletAbesupposedpossible。Then，whenAwaspossible，weagreed
thatnothingimpossiblefollowedifAweresupposedtobereal；and
thenBmustofcoursebereal。ButwesupposedBtobeimpossible。Let
itbeimpossiblethen。If，then，Bisimpossible，Aalsomustbeso。
Butthefirstwassupposedimpossible；thereforethesecondalsois
impossible。If，then，Aispossible，Balsowillbepossible，if
theyweresorelatedthatifA，isreal，Bmustbereal。If，then，A
andBbeingthusrelated，Bisnotpossibleonthiscondition，andB
willnotberelatedaswassupposed。AndifwhenAispossible，Bmust
bepossible，thenifAisreal，Balsomustbereal。FortosaythatB
mustbepossible，ifAispossible，meansthis，thatifAisrealboth
atthetimewhenandinthewayinwhichitwassupposedcapableof
beingreal，Balsomustthenandinthatwaybereal。