on the heavens-第19章

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　for　the　observed　facts　on　their　view　that　we　do　not　dwell　at　the　centre；　than　on　the　common　view　that　the　earth　is　in　the　middle。　Even　as　it　is；　there　is　nothing　in　the　observations　to　suggest　that　we　are　removed　from　the　centre　by　half　the　diameter　of　the　earth。　Others；　again；　say　that　the　earth；　which　lies　at　the　centre；　is　'rolled'；　and　thus　in　motion；　about　the　axis　of　the　whole　heaven；　So　it　stands　written　in　the　Timaeus。　　　III。　There　are　similar　disputes　about　the　shape　of　the　earth。　Some　think　it　is　spherical；　others　that　it　is　flat　and　drum…shaped。　For　evidence　they　bring　the　fact　that；　as　the　sun　rises　and　sets；　the　part　concealed　by　the　earth　shows　a　straight　and　not　a　curved　edge；　whereas　if　the　earth　were　spherical　the　line　of　section　would　have　to　be　circular。　In　this　they　leave　out　of　account　the　great　distance　of　the　sun　from　the　earth　and　the　great　size　of　the　circumference；　which；　seen　from　a　distance　on　these　apparently　small　circles　appears　straight。　Such　an　appearance　ought　not　to　make　them　doubt　the　circular　shape　of　the　earth。　But　they　have　another　argument。　They　say　that　because　it　is　at　rest；　the　earth　must　necessarily　have　this　shape。　For　there　are　many　different　ways　in　which　the　movement　or　rest　of　the　earth　has　been　conceived。　　　The　difficulty　must　have　occurred　to　every　one。　It　would　indeed　be　a　complacent　mind　that　felt　no　surprise　that；　while　a　little　bit　of　earth；　let　loose　in　mid…air　moves　and　will　not　stay　still；　and　more　there　is　of　it　the　faster　it　moves；　the　whole　earth；　free　in　midair；　should　show　no　movement　at　all。　Yet　here　is　this　great　weight　of　earth；　and　it　is　at　rest。　And　again；　from　beneath　one　of　these　moving　fragments　of　earth；　before　it　falls；　take　away　the　earth；　and　it　will　continue　its　downward　movement　with　nothing　to　stop　it。　The　difficulty　then；　has　naturally　passed　into　a　common　place　of　philosophy；　and　one　may　well　wonder　that　the　solutions　offered　are　not　seen　to　involve　greater　absurdities　than　the　problem　itself。　　　By　these　considerations　some　have　been　led　to　assert　that　the　earth　below　us　is　infinite；　saying；　with　Xenophanes　of　Colophon；　that　it　has　'pushed　its　roots　to　infinity'；…in　order　to　save　the　trouble　of　seeking　for　the　cause。　Hence　the　sharp　rebuke　of　Empedocles；　in　the　words　'if　the　deeps　of　the　earth　are　endless　and　endless　the　ample　ether…such　is　the　vain　tale　told　by　many　a　tongue；　poured　from　the　mouths　of　those　who　have　seen　but　little　of　the　whole。　Others　say　the　earth　rests　upon　water。　This；　indeed；　is　the　oldest　theory　that　has　been　preserved；　and　is　attributed　to　Thales　of　Miletus。　It　was　supposed　to　stay　still　because　it　floated　like　wood　and　other　similar　substances；　which　are　so　constituted　as　to　rest　upon　but　not　upon　air。　As　if　the　same　account　had　not　to　be　given　of　the　water　which　carries　the　earth　as　of　the　earth　itself！　It　is　not　the　nature　of　water；　any　more　than　of　earth；　to　stay　in　mid…air：　it　must　have　something　to　rest　upon。　Again；　as　air　is　lighter　than　water；　so　is　water　than　earth：　how　then　can　they　think　that　the　naturally　lighter　substance　lies　below　the　heavier？　Again；　if　the　earth　as　a　whole　is　capable　of　floating　upon　water；　that　must　obviously　be　the　case　with　any　part　of　it。　But　observation　shows　that　this　is　not　the　case。　Any　piece　of　earth　goes　to　the　bottom；　the　quicker　the　larger　it　is。　These　thinkers　seem　to　push　their　inquiries　some　way　into　the　problem；　but　not　so　far　as　they　might。　It　is　what　we　are　all　inclined　to　do；　to　direct　our　inquiry　not　by　the　matter　itself；　but　by　the　views　of　our　opponents：　and　even　when　interrogating　oneself　one　pushes　the　inquiry　only　to　the　point　at　which　one　can　no　longer　offer　any　opposition。　Hence　a　good　inquirer　will　be　one　who　is　ready　in　bringing　forward　the　objections　proper　to　the　genus；　and　that　he　will　be　when　he　has　gained　an　understanding　of　all　the　differences。　　　Anaximenes　and　Anaxagoras　and　Democritus　give　the　flatness　of　the　earth　as　the　cause　of　its　staying　still。　Thus；　they　say；　it　does　not　cut；　but　covers　like　a　lid；　the　air　beneath　it。　This　seems　to　be　the　way　of　flat…shaped　bodies：　for　even　the　wind　can　scarcely　move　them　because　of　their　power　of　resistance。　The　same　immobility；　they　say；　is　produced　by　the　flatness　of　the　surface　which　the　earth　presents　to　the　air　which　underlies　it；　while　the　air；　not　having　room　enough　to　change　its　place　because　it　is　underneath　the　earth；　stays　there　in　a　mass；　like　the　water　in　the　case　of　the　water…clock。　And　they　adduce　an　amount　of　evidence　to　prove　that　air；　when　cut　off　and　at　rest；　can　bear　a　considerable　weight。　　　Now；　first；　if　the　shape　of　the　earth　is　not　flat；　its　flatness　cannot　be　the　cause　of　its　immobility。　But　in　their　own　account　it　is　rather　the　size　of　the　earth　than　its　flatness　that　causes　it　to　remain　at　rest。　For　the　reason　why　the　air　is　so　closely　confined　that　it　cannot　find　a　passage；　and　therefore　stays　where　it　is；　is　its　great　amount：　and　this　amount　great　because　the　body　which　isolates　it；　the　earth；　is　very　large。　This　result；　then；　will　follow；　even　if　the　earth　is　spherical；　so　long　as　it　retains　its　size。　So　far　as　their　arguments　go；　the　earth　will　still　be　at　rest。　　　In　general；　our　quarrel　with　those　who　speak　of　movement　in　this　way　cannot　be　confined　to　the　parts；　it　concerns　the　whole　universe。　One　must　decide　at　the　outset　whether　bodies　have　a　natural　movement　or　not；　whether　there　is　no　natural　but　only　constrained　movement。　Seeing；　however；　that　we　have　already　decided　this　matter　to　the　best　of　our　ability；　we　are　entitled　to　treat　our　results　as　representing　fact。　Bodies；　we　say；　which　have　no　natural　movement；　have　no　constrained　movement；　and　where　there　is　no　natural　and　no　constrained　movement　there　will　be　no　movement　at　all。　This　is　a　conclusion；　the　necessity　of　which　we　have　already　decided；　and　we　have　seen　further　that　rest　also　will　be　inconceivable；　since　rest；　like　movement；　is　either　natural　or　constrained。　But　if　there　is　any　natural　movement；　constraint　will　not　be　the　sole　principle　of　motion　or　of　rest。　If；　then；　it　is　by　constraint　that　the　earth　now　keeps　its　place；　the　so…called　'whirling'　movement　by　which　its　parts　came　together　at　the　centre　was　also　constrained。　（The　form　of　causation　supposed　they　all　borrow　from　observations　of　liquids　and　of　air；　in　which　the　larger　and　heavier　bodies　always　move　to　the　centre　of　the　whirl。　This　is　thought　by　all　those　who　try　to　generate　the　heavens　to　explain　why　the　earth　came　together　at　the　centre。　They　then　seek　a　reason　for　its　staying　there；　and　some　say；　in　the　manner　explained；　that　the　reason　is　its　size　and　flatness；　others；　with　Empedocles；　that　the　motion　of　the　heavens；　moving　about　it　at　a　higher　speed；　prevents　movement　of　the　earth；　as　the　water　in　a　cup；　when　the　cup　is　given　a　circular　motion；　though　it　is　often　underneath　the　bronze；　is　for　this　same　reason　prevented　from　moving　with　the　downward　movement　which　is　natural　to　it。）　But　suppose　both　the　'whirl'　and　its　flatness　（the　air　beneath　being　withdrawn）　cease　to　prevent　the　earth's　motion；　where　will　the　earth　move　to　then？　Its　movement　to　the　centre　was　constrained；　and　its　rest　at　the　centre　is　due　to　constraint；　but　there　must　be　some　motion