g…point　or　at　the　goal　or　between　the　two；　and　we　expect　natural　motion　to　reach　its　maximum　at　the　goal；　unnatural　motion　at　the　starting…point；　and　missiles　midway　between　the　two。　But　circular　movement；　having　no　beginning　or　limit　or　middle　in　the　direct　sense　of　the　words；　has　neither　whence　nor　whither　nor　middle：　for　in　time　it　is　eternal；　and　in　length　it　returns　upon　itself　without　a　break。　If　then　its　movement　has　no　maximum；　it　can　have　no　irregularity；　since　irregularity　is　produced　by　retardation　and　acceleration。　Further；　since　everything　that　is　moved　is　moved　by　something；　the　cause　of　the　irregularity　of　movement　must　lie　either　in　the　mover　or　in　the　moved　or　both。　For　if　the　mover　moved　not　always　with　the　same　force；　or　if　the　moved　were　altered　and　did　not　remain　the　same；　or　if　both　were　to　change；　the　result　might　well　be　an　irregular　movement　in　the　moved。　But　none　of　these　possibilities　can　be　conceived　as　actual　in　the　case　of　the　heavens。　As　to　that　which　is　moved；　we　have　shown　that　it　is　primary　and　simple　and　ungenerated　and　indestructible　and　generally　unchanging；　and　the　mover　has　an　even　better　right　to　these　attributes。　It　is　the　primary　that　moves　the　primary；　the　simple　the　simple；　the　indestructible　and　ungenerated　that　which　is　indestructible　and　ungenerated。　Since　then　that　which　is　moved；　being　a　body；　is　nevertheless　unchanging；　how　should　the　mover；　which　is　incorporeal；　be　changed？　　　It　follows　then；　further；　that　the　motion　cannot　be　irregular。　For　if　irregularity　occurs；　there　must　be　change　either　in　the　movement　as　a　whole；　from　fast　to　slow　and　slow　to　fast；　or　in　its　parts。　That　there　is　no　irregularity　in　the　parts　is　obvious；　since；　if　there　were；　some　divergence　of　the　stars　would　have　taken　place　before　now　in　the　infinity　of　time；　as　one　moved　slower　and　another　faster：　but　no　alteration　of　their　intervals　is　ever　observed。　Nor　again　is　a　change　in　the　movement　as　a　whole　admissible。　Retardation　is　always　due　to　incapacity；　and　incapacity　is　unnatural。　The　incapacities　of　animals；　age；　decay；　and　the　like；　are　all　unnatural；　due；　it　seems；　to　the　fact　that　the　whole　animal　complex　is　made　up　of　materials　which　differ　in　respect　of　their　proper　places；　and　no　single　part　occupies　its　own　place。　If　therefore　that　which　is　primary　contains　nothing　unnatural；　being　simple　and　unmixed　and　in　its　proper　place　and　having　no　contrary；　then　it　has　no　place　for　incapacity；　nor；　consequently；　for　retardation　or　（since　acceleration　involves　retardation）　for　acceleration。　Again；　it　is　inconceivable　that　the　mover　should　first　show　incapacity　for　an　infinite　time；　and　capacity　afterwards　for　another　infinity。　For　clearly　nothing　which；　like　incapacity；　unnatural　ever　continues　for　an　infinity　of　time；　nor　does　the　unnatural　endure　as　long　as　the　natural；　or　any　form　of　incapacity　as　long　as　the　capacity。　But　if　the　movement　is　retarded　it　must　necessarily　be　retarded　for　an　infinite　time。　Equally　impossible　is　perpetual　acceleration　or　perpetual　retardation。　For　such　movement　would　be　infinite　and　indefinite；　but　every　movement；　in　our　view；　proceeds　from　one　point　to　another　and　is　definite　in　character。　Again；　suppose　one　assumes　a　minimum　time　in　less　than　which　the　heaven　could　not　complete　its　movement。　For；　as　a　given　walk　or　a　given　exercise　on　the　harp　cannot　take　any　and　every　time；　but　every　performance　has　its　definite　minimum　time　which　is　unsurpassable；　so；　one　might　suppose；　the　movement　of　the　heaven　could　not　be　completed　in　any　and　every　time。　But　in　that　case　perpetual　acceleration　is　impossible　（and；　equally；　perpetual　retardation：　for　the　argument　holds　of　both　and　each）；　if　we　may　take　acceleration　to　proceed　by　identical　or　increasing　additions　of　speed　and　for　an　infinite　time。　The　remaining　alternative　is　to　say　that　the　movement　exhibits　an　alternation　of　slower　and　faster：　but　this　is　a　mere　fiction　and　quite　inconceivable。　Further；　irregularity　of　this　kind　would　be　particularly　unlikely　to　pass　unobserved；　since　contrast　makes　observation　easy。　　　That　there　is　one　heaven；　then；　only；　and　that　it　is　ungenerated　and　eternal；　and　further　that　its　movement　is　regular；　has　now　been　sufficiently　explained。
  7
  We　have　next　to　speak　of　the　stars；　as　they　are　called；　of　their　composition；　shape；　and　movements。　It　would　be　most　natural　and　consequent　upon　what　has　been　said　that　each　of　the　stars　should　be　composed　of　that　substance　in　which　their　path　lies；　since；　as　we　said；　there　is　an　element　whose　natural　movement　is　circular。　In　so　saying　we　are　only　following　the　same　line　of　thought　as　those　who　say　that　the　stars　are　fiery　because　they　believe　the　upper　body　to　be　fire；　the　presumption　being　that　a　thing　is　composed　of　the　same　stuff　as　that　in　which　it　is　situated。　The　warmth　and　light　which　proceed　from　them　are　caused　by　the　friction　set　up　in　the　air　by　their　motion。　Movement　tends　to　create　fire　in　wood；　stone；　and　iron；　and　with　even　more　reason　should　it　have　that　effect　on　air；　a　substance　which　is　closer　to　fire　than　these。　An　example　is　that　of　missiles；　which　as　they　move　are　themselves　fired　so　strongly　that　leaden　balls　are　melted；　and　if　they　are　fired　the　surrounding　air　must　be　similarly　affected。　Now　while　the　missiles　are　heated　by　reason　of　their　motion　in　air；　which　is　turned　into　fire　by　the　agitation　produced　by　their　movement；　the　upper　bodies　are　carried　on　a　moving　sphere；　so　that；　though　they　are　not　themselves　fired；　yet　the　air　underneath　the　sphere　of　the　revolving　body　is　necessarily　heated　by　its　motion；　and　particularly　in　that　part　where　the　sun　is　attached　to　it。　Hence　warmth　increases　as　the　sun　gets　nearer　or　higher　or　overhead。　Of　the　fact；　then；　that　the　stars　are　neither　fiery　nor　move　in　fire；　enough　has　been　said。
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  Since　changes　evidently　occur　not　only　in　the　position　of　the　stars　but　also　in　that　of　the　whole　heaven；　there　are　three　possibilities。　Either　（1）　both　are　at　rest；　or　（2）　both　are　in　motion；　or　（3）　the　one　is　at　rest　and　the　other　in　motion。　　　（1）　That　both　should　be　at　rest　is　impossible；　for；　if　the　earth　is　at　rest；　the　hypothesis　does　not　account　for　the　observations；　and　we　take　it　as　granted　that　the　earth　is　at　rest。　It　remains　either　that　both　are　moved；　or　that　the　one　is　moved　and　the　other　at　rest。　　　（2）　On　the　view；　first；　that　both　are　in　motion；　we　have　the　absurdity　that　the　stars　and　the　circles　move　with　the　same　speed；　i。e。　that　the　ace　of　every　star　is　that　of　the　circle　in　it　moves。　For　star　and　circle　are　seen　to　come　back　to　the　same　place　at　the　same　moment；　from　which　it　follows　that　the　star　has　traversed　the　circle　and　the　circle　has　completed　its　own　movement；　i。e。　traversed　its　own　circumference；　at　one　and　the　same　moment。　But　it　is　difficult　to　conceive　that　the　pace　of　each　star　should　be　exactly　proportioned　to　the　size　of　its　circle。　That　the　pace　of　each　circle　should　be　proportionate　to　its　size　is　not　absurd　but　inevitable：　but　that　the　same　should　be　true　of　the　movement　of　the　stars　contained　in　the　circles　is　quite　incredible。　For　if；　on　the　one　and；　we　suppose　that　the　star　which　moves　on　the　greater　circle　is　necessarily　swifter；　clearly　we　also　admit　that　if　stars　shifted　their　position　so　as　to　exchange　circles；　the　slower　would　become　swifter　and　the　swifter　slower。　But　this　would　show　that　their　movement　was　not　their　own；　but　due　to　the　circles。　If；　on　the　other　hand；　the　arrangement　was　a　chance　combination；　the　coincidence　in　every　case　of　a　greater　circle　with　a　swifter　movement　of　the　star　contained　in　it　is　too　much　to　believe。　In　one　or　two　cases　it　might　not　inconceivably　fall　out　so；　but　to　imagine　it　in　every　case　alike　is　a　mere　fiction。　Besides；　chance　has　no　place　in　that　which　is　natural；　and　what　happens　everywhere　and　in　every　case　is　no　matter　of　chance。　　　（3）　The　same　absurdity　is　equally　plain　if　it　is　supposed　that　the　circles　stand　still　and　that　it　is　the　stars　themselves　which　move。　For　it　will　follow　that　the　outer　stars　are　the　swifter；　and　that　the　pace　of　the　stars　corresponds　to　the　size　of　their　circles。　　　Since；　then；　we　cannot　reasonably　suppose　either　that　both　are　in　motion　or　that　the　star　alone　moves；　the　remaining　alternative　is　that　the　circles　should　move；　while　the　stars　are　at　rest　and　move　with　the　circles　to　which　they　are　attached。　Only　on　this　supposition　are　we　involved　in　no　absurd　consequence。　For；　in　the　first　place；　the　quicker　movement　of　the　larger　circle　is　natural　when　all　the　circles　are　attached　to　the　same　centre。　Whenever　bodies　are　moving　with　their　proper　motion；　the　larger　moves　quicker。　It　is　the　same　here　with　the　revolving　bodies：　for　the　are　intercepted　by　two　radii　will　be　larger　in　the　larger　circle；　and　hence　it　is　not　surprising　that　the　revolution　of　the　larger　circle　should　take　the　same　time　as　that　of　the　smaller。　And　secondly；　the　fact　that　the　heavens　do　not　break　in　pieces　follows　not　only　from　this　but　also　from　the　proof　already　given　of　the　continuity　of　the　whole。　　　Again；　since　the　stars　are　spherical；　as　our　opponents　assert　and　we　may　consistently　admit；　inasmuch　as　we　construct　them　out　of　the　spherical　body；　and