ing　to　which　the　passage　from　not　being　to　being　belongs；　whether　already　actual；　if　its　existence　is　due　to　a　past　process　of　becoming；　or　not　yet　actual　but　only　possible。　The　uses　of　the　words　'destructible'　and　'indestructible'　are　similar。　'Destructible'　is　applied　（a）　to　that　which　formerly　was　and　afterwards　either　is　not　or　might　not　be；　whether　a　period　of　being　destroyed　and　changed　intervenes　or　not；　and　（b）　sometimes　we　apply　the　word　to　that　which　a　process　of　destruction　may　cause　not　to　be；　and　also　（c）　in　a　third　sense；　to　that　which　is　easily　destructible；　to　the　'easily　destroyed'；　so　to　speak。　Of　the　indestructible　the　same　account　holds　good。　It　is　either　（a）　that　which　now　is　and　now　is　not；　without　any　process　of　destruction；　like　contact；　which　without　being　destroyed　afterwards　is　not；　though　formerly　it　was；　or　（b）　that　which　is　but　might　not　be；　or　which　will　at　some　time　not　be；　though　it　now　is。　For　you　exist　now　and　so　does　the　contact；　yet　both　are　destructible；　because　a　time　will　come　when　it　will　not　be　true　of　you　that　you　exist；　nor　of　these　things　that　they　are　in　contact。　Thirdly　（c）　in　its　most　proper　use；　it　is　that　which　is；　but　is　incapable　of　any　destruction　such　that　the　thing　which　now　is　later　ceases　to　be　or　might　cease　to　be；　or　again；　that　which　has　not　yet　been　destroyed；　but　in　the　future　may　cease　to　be。　For　indestructible　is　also　used　of　that　which　is　destroyed　with　difficulty。　　　This　being　so；　we　must　ask　what　we　mean　by　'possible'　and　'impossible'。　For　in　its　most　proper　use　the　predicate　'indestructible'　is　given　because　it　is　impossible　that　the　thing　should　be　destroyed；　i。e。　exist　at　one　time　and　not　at　another。　And　'ungenerated'　also　involves　impossibility　when　used　for　that　which　cannot　be　generated；　in　such　fashion　that；　while　formerly　it　was　not；　later　it　is。　An　instance　is　a　commensurable　diagonal。　Now　when　we　speak　of　a　power　to　move　or　to　lift　weights；　we　refer　always　to　the　maximum。　We　speak；　for　instance；　of　a　power　to　lift　a　hundred　talents　or　walk　a　hundred　stades…though　a　power　to　effect　the　maximum　is　also　a　power　to　effect　any　part　of　the　maximum…since　we　feel　obliged　in　defining　the　power　to　give　the　limit　or　maximum。　A　thing；　then；　which　is　within　it。　If；　for　example；　a　man　can　lift　a　hundred　talents；　he　can　also　lift　two；　and　if　he　can　walk　a　hundred　stades；　he　can　also　walk　two。　But　the　power　is　of　the　maximum；　and　a　thing　said；　with　reference　to　its　maximum；　to　be　incapable　of　so　much　is　also　incapable　of　any　greater　amount。　It　is；　for　instance；　clear　that　a　person　who　cannot　walk　a　thousand　stades　will　also　be　unable　to　walk　a　thousand　and　one。　This　point　need　not　trouble　us；　for　we　may　take　it　as　settled　that　what　is；　in　the　strict　sense；　possible　is　determined　by　a　limiting　maximum。　Now　perhaps　the　objection　might　be　raised　that　there　is　no　necessity　in　this；　since　he　who　sees　a　stade　need　not　see　the　smaller　measures　contained　in　it；　while；　on　the　contrary；　he　who　can　see　a　dot　or　hear　a　small　sound　will　perceive　what　is　greater。　This；　however；　does　not　touch　our　argument。　The　maximum　may　be　determined　either　in　the　power　or　in　its　object。　The　application　of　this　is　plain。　Superior　sight　is　sight　of　the　smaller　body；　but　superior　speed　is　that　of　the　greater　body。
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  Having　established　these　distinctions　we　car　now　proceed　to　the　sequel。　If　there　are　thing！　capable　both　of　being　and　of　not　being；　there　must　be　some　definite　maximum　time　of　their　being　and　not　being；　a　time；　I　mean；　during　which　continued　existence　is　possible　to　them　and　a　time　during　which　continued　nonexistence　is　possible。　And　this　is　true　in　every　category；　whether　the　thing　is；　for　example；　'man'；　or　'white'；　or　'three　cubits　long'；　or　whatever　it　may　be。　For　if　the　time　is　not　definite　in　quantity；　but　longer　than　any　that　can　be　suggested　and　shorter　than　none；　then　it　will　be　possible　for　one　and　the　same　thing　to　exist　for　infinite　time　and　not　to　exist　for　another　infinity。　This；　however；　is　impossible。　　　Let　us　take　our　start　from　this　point。　The　impossible　and　the　false　have　not　the　same　significance。　One　use　of　'impossible'　and　'possible'；　and　'false'　and　'true'；　is　hypothetical。　It　is　impossible；　for　instance；　on　a　certain　hypothesis　that　the　triangle　should　have　its　angles　equal　to　two　right　angles；　and　on　another　the　diagonal　is　commensurable。　But　there　are　also　things　possible　and　impossible；　false　and　true；　absolutely。　Now　it　is　one　thing　to　be　absolutely　false；　and　another　thing　to　be　absolutely　impossible。　To　say　that　you　are　standing　when　you　are　not　standing　is　to　assert　a　falsehood；　but　not　an　impossibility。　Similarly　to　say　that　a　man　who　is　playing　the　harp；　but　not　singing；　is　singing；　is　to　say　what　is　false　but　not　impossible。　To　say；　however；　that　you　are　at　once　standing　and　sitting；　or　that　the　diagonal　is　commensurable；　is　to　say　what　is　not　only　false　but　also　impossible。　Thus　it　is　not　the　same　thing　to　make　a　false　and　to　make　an　impossible　hypothesis；　and　from　the　impossible　hypothesis　impossible　results　follow。　A　man　has；　it　is　true；　the　capacity　at　once　of　sitting　and　of　standing；　because　when　he　possesses　the　one　he　also　possesses　the　other；　but　it　does　not　follow　that　he　can　at　once　sit　and　stand；　only　that　at　another　time　he　can　do　the　other　also。　But　if　a　thing　has　for　infinite　time　more　than　one　capacity；　another　time　is　impossible　and　the　times　must　coincide。　Thus　if　a　thing　which　exists　for　infinite　time　is　destructible；　it　will　have　the　capacity　of　not　being。　Now　if　it　exists　for　infinite　time　let　this　capacity　be　actualized；　and　it　will　be　in　actuality　at　once　existent　and　non…existent。　Thus　a　false　conclusion　would　follow　because　a　false　assumption　was　made；　but　if　what　was　assumed　had　not　been　impossible　its　consequence　would　not　have　been　impossible。　　　Anything　then　which　always　exists　is　absolutely　imperishable。　It　is　also　ungenerated；　since　if　it　was　generated　it　will　have　the　power　for　some　time　of　not　being。　For　as　that　which　formerly　was；　but　now　is　not；　or　is　capable　at　some　future　time　of　not　being；　is　destructible；　so　that　which　is　capable　of　formerly　not　having　been　is　generated。　But　in　the　case　of　that　which　always　is；　there　is　no　time　for　such　a　capacity　of　not　being；　whether　the　supposed　time　is　finite　or　infinite；　for　its　capacity　of　being　must　include　the　finite　time　since　it　covers　infinite　time。　　　It　is　therefore　impossible　that　one　and　the　same　thing　should　be　capable　of　always　existing　and　of　always　not…existing。　And　'not　always　existing'；　the　contradictory；　is　also　excluded。　Thus　it　is　impossible　for　a　thing　always　to　exist　and　yet　to　be　destructible。　Nor；　similarly；　can　it　be　generated。　For　of　two　attributes　if　B　cannot　be　present　without　A；　the　impossibility　A　of　proves　the　impossibility　of　B。　What　always　is；　then；　since　it　is　incapable　of　ever　not　being；　cannot　possibly　be　generated。　But　since　the　contradictory　of　'that　which　is　always　capable　of　being'　'that　which　is　not　always　capable　of　being'；　while　'that　which　is　always　capable　of　not　being'　is　the　contrary；　whose　contradictory　in　turn　is　'that　which　is　not　always　capable　of　not　being'；　it　is　necessary　that　the　contradictories　of　both　terms　should　be　predicable　of　one　and　the　same　thing；　and　thus　that；　intermediate　between　what　always　is　and　what　always　is　not；　there　should　be　that　to　which　being　and　not…being　are　both　possible；　for　the　contradictory　of　each　will　at　times　be　true　of　it　unless　it　always　exists。　Hence　that　which　not　always　is　not　will　sometimes　be　and　sometimes　not　be；　and　it　is　clear　that　this　is　true　also　of　that　which　cannot　always　be　but　sometimes　is　and　therefore　sometimes　is　not。　One　thing；　then；　will　have　the　power　of　being；　and　will　thus　be　intermediate　between　the　other　two。　　　Expresed　universally　our　argument　is　as　follows。　Let　there　be　two　attributes；　A　and　B；　not　capable　of　being　present　in　any　one　thing　together；　while　either　A　or　C　and　either　B　or　D　are　capable　of　being　present　in　everything。　Then　C　and　D　must　be　predicated　of　everything　of　which　neither　A　nor　B　is　predicated。　Let　E　lie　between　A　and　B；　for　that　which　is　neither　of　two　contraries　is　a　mean　between　them。　In　E　both　C　and　D　must　be　present；　for　either　A　or　C　is　present　everywhere　and　therefore　in　E。　Since　then　A　is　impossible；　C　must　be　present；　and　the　same　argument　holds　of　D。　　　Neither　that　which　always　is；　therefore；　nor　that　which　always　is　not　is　either　generated　or　destructible。　And　clearly　whatever　is　generated　or　destructible　is　not　eternal。　If　it　were；　it　would　be　at　once　capable　of　always　being　and　capable　of　not　always　being；　but　it　has　already　been　shown　that　this　is　impossible。　Surely　then　whatever　is　ungenerated　and　in　being　must　be　eternal；　and　whatever　is　indestructible　and　in　being　must　equally　be　so。　（I　use　the　words　'ungenerated'　and　'indestructible'　in　their　proper　sense；　'ungenerated'　for　that　which　now　is　and　could　not　at　any　previous　time　have　been　truly　said　not　to　be；　'indestructible'　for　that　which　now　is　and　cannot　at　any　future　time　be　truly　said　not　to　be。）　If；　again；　the　two　terms　are　coincident；　if　the　ungenerated　is　indestructible；　and　the　indestructible　ungenearted；　then　each　of　them　is　coincident　with