so　it　must　needs　remain　still。　This　view　is　ingenious　but　not　true。　The　argument　would　prove　that　everything；　whatever　it　be；　which　is　put　at　the　centre；　must　stay　there。　Fire；　then；　will　rest　at　the　centre：　for　the　proof　turns　on　no　peculiar　property　of　earth。　But　this　does　not　follow。　The　observed　facts　about　earth　are　not　only　that　it　remains　at　the　centre；　but　also　that　it　moves　to　the　centre。　The　place　to　which　any　fragment　of　earth　moves　must　necessarily　be　the　place　to　which　the　whole　moves；　and　in　the　place　to　which　a　thing　naturally　moves；　it　will　naturally　rest。　The　reason　then　is　not　in　the　fact　that　the　earth　is　indifferently　related　to　every　extreme　point：　for　this　would　apply　to　any　body；　whereas　movement　to　the　centre　is　peculiar　to
  earth。　Again　it　is　absurd　to　look　for　a　reason　why　the　earth　remains　at　the　centre　and　not　for　a　reason　why　fire　remains　at　the　extremity。　If　the　extremity　is　the　natural　place　of　fire；　clearly　earth　must　also　have　a　natural　place。　But　suppose　that　the　centre　is　not　its　place；　and　that　the　reason　of　its　remaining　there　is　this　necessity　of　indifference…on　the　analogy　of　the　hair　which；　it　is　said；　however　great　the　tension；　will　not　break　under　it；　if　it　be　evenly　distributed；　or　of　the　men　who；　though　exceedingly　hungry　and　thirsty；　and　both　equally；　yet　being　equidistant　from　food　and　drink；　is　therefore　bound　to　stay　where　he　is…even　so；　it　still　remains　to　explain　why　fire　stays　at　the　extremities。　It　is　strange；　too；　to　ask　about　things　staying　still　but　not　about　their　motion；…why；　I　mean；　one　thing；　if　nothing　stops　it；　moves　up；　and　another　thing　to　the　centre。　Again；　their　statements　are　not　true。　It　happens；　indeed；　to　be　the　case　that　a　thing　to　which　movement　this　way　and　that　is　equally　inappropriate　is　obliged　to　remain　at　the　centre。　But　so　far　as　their　argument　goes；　instead　of　remaining　there；　it　will　move；　only　not　as　a　mass　but　in　fragments。　For　the　argument　applies　equally　to　fire。　Fire；　if　set　at　the　centre；　should　stay　there；　like　earth；　since　it　will　be　indifferently　related　to　every　point　on　the　extremity。　Nevertheless　it　will　move；　as　in　fact　it　always　does　move　when　nothing　stops　it；　away　from　the　centre　to　the　extremity。　It　will　not；　however；　move　in　a　mass　to　a　single　point　on　the　circumference…the　only　possible　result　on　the　lines　of　the　indifference　theory…but　rather　each　corresponding　portion　of　fire　to　the　corresponding　part　of　the　extremity；　each　fourth　part；　for　instance；　to　a　fourth　part　of　the　circumference。　For　since　no　body　is　a　point；　it　will　have　parts。　The　expansion；　when　the　body　increased　the　place　occupied；　would　be　on　the　same　principle　as　the　contraction；　in　which　the　place　was　diminished。　Thus；　for　all　the　indifference　theory　shows　to　the　contrary；　earth　also　would　have　moved　in　this　manner　away　from　the　centre；　unless　the　centre　had　been　its　natural　place。　　　We　have　now　outlined　the　views　held　as　to　the　shape；　position；　and　rest　or　movement　of　the　earth。
  14
  Let　us　first　decide　the　question　whether　the　earth　moves　or　is　at　rest。　For；　as　we　said；　there　are　some　who　make　it　one　of　the　stars；　and　others　who；　setting　it　at　the　centre；　suppose　it　to　be　'rolled'　and　in　motion　about　the　pole　as　axis。　That　both　views　are　untenable　will　be　clear　if　we　take　as　our　starting…point　the　fact　that　the　earth's　motion；　whether　the　earth　be　at　the　centre　or　away　from　it；　must　needs　be　a　constrained　motion。　It　cannot　be　the　movement　of　the　earth　itself。　If　it　were；　any　portion　of　it　would　have　this　movement；　but　in　fact　every　part　moves　in　a　straight　line　to　the　centre。　Being；　then；　constrained　and　unnatural；　the　movement　could　not　be　eternal。　But　the　order　of　the　universe　is　eternal。　Again；　everything　that　moves　with　the　circular　movement；　except　the　first　sphere；　is　observed　to　be　passed；　and　to　move　with　more　than　one　motion。　The　earth；　then；　also；　whether　it　move　about　the　centre　or　as　stationary　at　it；　must　necessarily　move　with　two　motions。　But　if　this　were　so；　there　would　have　to　be　passings　and　turnings　of　the　fixed　stars。　Yet　no　such　thing　is　observed。　The　same　stars　always　rise　and　set　in　the　same　parts　of　the　earth。　　　Further；　the　natural　movement　of　the　earth；　part　and　whole　alike；　is　the　centre　of　the　whole…whence　the　fact　that　it　is　now　actually　situated　at　the　centre…but　it　might　be　questioned　since　both　centres　are　the　same；　which　centre　it　is　that　portions　of　earth　and　other　heavy　things　move　to。　Is　this　their　goal　because　it　is　the　centre　of　the　earth　or　because　it　is　the　centre　of　the　whole？　The　goal；　surely；　must　be　the　centre　of　the　whole。　For　fire　and　other　light　things　move　to　the　extremity　of　the　area　which　contains　the　centre。　It　happens；　however；　that　the　centre　of　the　earth　and　of　the　whole　is　the　same。　Thus　they　do　move　to　the　centre　of　the　earth；　but　accidentally；　in　virtue　of　the　fact　that　the　earth's　centre　lies　at　the　centre　of　the　whole。　That　the　centre　of　the　earth　is　the　goal　of　their　movement　is　indicated　by　the　fact　that　heavy　bodies　moving　towards　the　earth　do　not　parallel　but　so　as　to　make　equal　angles；　and　thus　to　a　single　centre；　that　of　the　earth。　It　is　clear；　then；　that　the　earth　must　be　at　the　centre　and　immovable；　not　only　for　the　reasons　already　given；　but　also　because　heavy　bodies　forcibly　thrown　quite　straight　upward　return　to　the　point　from　which　they　started；　even　if　they　are　thrown　to　an　infinite　distance。　From　these　considerations　then　it　is　clear　that　the　earth　does　not　move　and　does　not　lie　elsewhere　than　at　the　centre。　　　From　what　we　have　said　the　explanation　of　the　earth's　immobility　is　also　apparent。　If　it　is　the　nature　of　earth；　as　observation　shows；　to　move　from　any　point　to　the　centre；　as　of　fire　contrariwise　to　move　from　the　centre　to　the　extremity；　it　is　impossible　that　any　portion　of　earth　should　move　away　from　the　centre　except　by　constraint。　For　a　single　thing　has　a　single　movement；　and　a　simple　thing　a　simple：　contrary　movements　cannot　belong　to　the　same　thing；　and　movement　away　from　the　centre　is　the　contrary　of　movement　to　it。　If　then　no　portion　of　earth　can　move　away　from　the　centre；　obviously　still　less　can　the　earth　as　a　whole　so　move。　For　it　is　the　nature　of　the　whole　to　move　to　the　point　to　which　the　part　naturally　moves。　Since；　then；　it　would　require　a　force　greater　than　itself　to　move　it；　it　must　needs　stay　at　the　centre。　This　view　is　further　supported　by　the　contributions　of　mathematicians　to　astronomy；　since　the　observations　made　as　the　shapes　change　by　which　the　order　of　the　stars　is　determined；　are　fully　accounted　for　on　the　hypothesis　that　the　earth　lies　at　the　centre。　Of　the　position　of　the　earth　and　of　the　manner　of　its　rest　or　movement；　our　discussion　may　here　end。　　　Its　shape　must　necessarily　be　spherical。　For　every　portion　of　earth　has　weight　until　it　reaches　the　centre；　and　the　jostling　of　parts　greater　and　smaller　would　bring　about　not　a　waved　surface；　but　rather　compression　and　convergence　of　part　and　part　until　the　centre　is　reached。　The　process　should　be　conceived　by　supposing　the　earth　to　come　into　being　in　the　way　that　some　of　the　natural　philosophers　describe。　Only　they　attribute　the　downward　movement　to　constraint；　and　it　is　better　to　keep　to　the　truth　and　say　that　the　reason　of　this　motion　is　that　a　thing　which　possesses　weight　is　naturally　endowed　with　a　centripetal　movement。　When　the　mixture；　then；　was　merely　potential；　the　things　that　were　separated　off　moved　similarly　from　every　side　towards　the　centre。　Whether　the　parts　which　came　together　at　the　centre　were　distributed　at　the　extremities　evenly；　or　in　some　other　way；　makes　no　difference。　If；　on　the　one　hand；　there　were　a　similar　movement　from　each　quarter　of　the　extremity　to　the　single　centre；　it　is　obvious　that　the　resulting　mass　would　be　similar　on　every　side。　For　if　an　equal　amount　is　added　on　every　side　the　extremity　of　the　mass　will　be　everywhere　equidistant　from　its　centre；　i。e。　the　figure　will　be　spherical。　But　neither　will　it　in　any　way　affect　the　argument　if　there　is　not　a　similar　accession　of　concurrent　fragments　from　every　side。　For　the　greater　quantity；　finding　a　lesser　in　front　of　it；　must　necessarily　drive　it　on；　both　having　an　impulse　whose　goal　is　the　centre；　and　the　greater　weight　driving　the　lesser　forward　till　this　goal　is　reached。　In　this　we　have　also　the　solution　of　a　possible　difficulty。　The　earth；　it　might　be　argued；　is　at　the　centre　and　spherical　in　shape：　if；　then；　a　weight　many　times　that　of　the　earth　were　added　to　one　hemisphere；　the　centre　of　the　earth　and　of　the　whole　will　no　longer　be　coincident。　So　that　either　the　earth　will　not　stay　still　at　the　centre；　or　if　it　does；　it　will　be　at　rest　without　having　its　centre　at　the　place　to　which　it　is　still　its　nature　to　move。　Such　is　the　difficulty。　A　short　consideration　will　give　us　an　easy　answer；　if　we　first　give　precision　to　our　postulate　that　any　body　endowed　with　weight；　of　whatever　size；　moves　towards　the　centre。　Clearly　it　will　not　stop　when　its　edge　touches　the　centre。　The　greater　quantity　must　prevail　until　the　body's　centre　occupies　the　centre。　For　that　is　the　goal　of　its　impulse。　Now　it　makes　no　difference　whether　we　apply　this　to　a　clod　or　common　fragment　of　earth　or　to　the　earth　as　a　whole。