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。