ON　THE　HEAVENS
  by　Aristotle
  translated　by　J。　L。　Stocks
  Book　I
  1
  THE　science　which　has　to　do　with　nature　clearly　concerns　itself　for　the　most　part　with　bodies　and　magnitudes　and　their　properties　and　movements；　but　also　with　the　principles　of　this　sort　of　substance；　as　many　as　they　may　be。　For　of　things　constituted　by　nature　some　are　bodies　and　magnitudes；　some　possess　body　and　magnitude；　and　some　are　principles　of　things　which　possess　these。　Now　a　continuum　is　that　which　is　divisible　into　parts　always　capable　of　subdivision；　and　a　body　is　that　which　is　every　way　divisible。　A　magnitude　if　divisible　one　way　is　a　line；　if　two　ways　a　surface；　and　if　three　a　body。　Beyond　these　there　is　no　other　magnitude；　because　the　three　dimensions　are　all　that　there　are；　and　that　which　is　divisible　in　three　directions　is　divisible　in　all。　For；　as　the　Pythagoreans　say；　the　world　and　all　that　is　in　it　is　determined　by　the　number　three；　since　beginning　and　middle　and　end　give　the　number　of　an　'all'；　and　the　number　they　give　is　the　triad。　And　so；　having　taken　these　three　from　nature　as　（so　to　speak）　laws　of　it；　we　make　further　use　of　the　number　three　in　the　worship　of　the　Gods。　Further；　we　use　the　terms　in　practice　in　this　way。　Of　two　things；　or　men；　we　say　'both'；　but　not　'all'：　three　is　the　first　number　to　which　the　term　'all'　has　been　appropriated。　And　in　this；　as　we　have　said；　we　do　but　follow　the　lead　which　nature　gives。　Therefore；　since　'every'　and　'all'　and　'complete'　do　not　differ　from　one　another　in　respect　of　form；　but　only；　if　at　all；　in　their　matter　and　in　that　to　which　they　are　applied；　body　alone　among　magnitudes　can　be　complete。　For　it　alone　is　determined　by　the　three　dimensions；　that　is；　is　an　'all'。　But　if　it　is　divisible　in　three　dimensions　it　is　every　way　divisible；　while　the　other　magnitudes　are　divisible　in　one　dimension　or　in　two　alone：　for　the　divisibility　and　continuity　of　magnitudes　depend　upon　the　number　of　the　dimensions；　one　sort　being　continuous　in　one　direction；　another　in　two；　another　in　all。　All　magnitudes；　then；　which　are　divisible　are　also　continuous。　Whether　we　can　also　say　that　whatever　is　continuous　is　divisible　does　not　yet；　on　our　present　grounds；　appear。　One　thing；　however；　is　clear。　We　cannot　pass　beyond　body　to　a　further　kind；　as　we　passed　from　length　to　surface；　and　from　surface　to　body。　For　if　we　could；　it　would　cease　to　be　true　that　body　is　complete　magnitude。　We　could　pass　beyond　it　only　in　virtue　of　a　defect　in　it；　and　that　which　is　complete　cannot　be　defective；　since　it　has　being　in　every　respect。　Now　bodies　which　are　classed　as　parts　of　the　whole　are　each　complete　according　to　our　formula；　since　each　possesses　every　dimension。　But　each　is　determined　relatively　to　that　part　which　is　next　to　it　by　contact；　for　which　reason　each　of　them　is　in　a　sense　many　bodies。　But　the　whole　of　which　they　are　parts　must　necessarily　be　complete；　and　thus；　in　accordance　with　the　meaning　of　the　word；　have　being；　not　in　some　respect　only；　but　in　every　respect。
  2
  The　question　as　to　the　nature　of　the　whole；　whether　it　is　infinite　in　size　or　limited　in　its　total　mass；　is　a　matter　for　subsequent　inquiry。　We　will　now　speak　of　those　parts　of　the　whole　which　are　specifically　distinct。　Let　us　take　this　as　our　starting…point。　All　natural　bodies　and　magnitudes　we　hold　to　be；　as　such；　capable　of　locomotion；　for　nature；　we　say；　is　their　principle　of　movement。　But　all　movement　that　is　in　place；　all　locomotion；　as　we　term　it；　is　either　straight　or　circular　or　a　combination　of　these　two；　which　are　the　only　simple　movements。　And　the　reason　of　this　is　that　these　two；　the　straight　and　the　circular　line；　are　the　only　simple　magnitudes。　Now　revolution　about　the　centre　is　circular　motion；　while　the　upward　and　downward　movements　are　in　a　straight　line；　'upward'　meaning　motion　away　from　the　centre；　and　'downward'　motion　towards　it。　All　simple　motion；　then；　must　be　motion　either　away　from　or　towards　or　about　the　centre。　This　seems　to　be　in　exact　accord　with　what　we　said　above：　as　body　found　its　completion　in　three　dimensions；　so　its　movement　completes　itself　in　three　forms。　　　Bodies　are　either　simple　or　compounded　of　such；　and　by　simple　bodies　I　mean　those　which　possess　a　principle　of　movement　in　their　own　nature；　such　as　fire　and　earth　with　their　kinds；　and　whatever　is　akin　to　them。　Necessarily；　then；　movements　also　will　be　either　simple　or　in　some　sort　compound…simple　in　the　case　of　the　simple　bodies；　compound　in　that　of　the　composite…and　in　the　latter　case　the　motion　will　be　that　of　the　simple　body　which　prevails　in　the　composition。　Supposing；　then；　that　there　is　such　a　thing　as　simple　movement；　and　that　circular　movement　is　an　instance　of　it；　and　that　both　movement　of　a　simple　body　is　simple　and　simple　movement　is　of　a　simple　body　（for　if　it　is　movement　of　a　compound　it　will　be　in　virtue　of　a　prevailing　simple　element）；　then　there　must　necessarily　be　some　simple　body　which　revolves　naturally　and　in　virtue　of　its　own　nature　with　a　circular　movement。　By　constraint；　of　course；　it　may　be　brought　to　move　with　the　motion　of　something　else　different　from　itself；　but　it　cannot　so　move　naturally；　since　there　is　one　sort　of　movement　natural　to　each　of　the　simple　bodies。　Again；　if　the　unnatural　movement　is　the　contrary　of　the　natural　and　a　thing　can　have　no　more　than　one　contrary；　it　will　follow　that　circular　movement；　being　a　simple　motion；　must　be　unnatural；　if　it　is　not　natural；　to　the　body　moved。　If　then　（1）　the　body；　whose　movement　is　circular；　is　fire　or　some　other　element；　its　natural　motion　must　be　the　contrary　of　the　circular　motion。　But　a　single　thing　has　a　single　contrary；　and　upward　and　downward　motion　are　the　contraries　of　one　another。　If；　on　the　other　hand；　（2）　the　body　moving　with　this　circular　motion　which　is　unnatural　to　it　is　something　different　from　the　elements；　there　will　be　some　other　motion　which　is　natural　to　it。　But　this　cannot　be。　For　if　the　natural　motion　is　upward；　it　will　be　fire　or　air；　and　if　downward；　water　or　earth。　Further；　this　circular　motion　is　necessarily　primary。　For　the　perfect　is　naturally　prior　to　the　imperfect；　and　the　circle　is　a　perfect　thing。　This　cannot　be　said　of　any　straight　line：…not　of　an　infinite　line；　for；　if　it　were　perfect；　it　would　have　a　limit　and　an　end：　nor　of　any　finite　line；　for　in　every　case　there　is　something　beyond　it；　since　any　finite　line　can　be　extended。　And　so；　since　the　prior　movement　belongs　to　the　body　which　naturally　prior；　and　circular　movement　is　prior　to　straight；　and　movement　in　a　straight　line　belongs　to　simple　bodies…fire　moving　straight　upward　and　earthy　bodies　straight　downward　towards　the　centre…since　this　is　so；　it　follows　that　circular　movement　also　must　be　the　movement　of　some　simple　body。　For　the　movement　of　composite　bodies　is；　as　we　said；　determined　by　that　simple　body　which　preponderates　in　the　composition。　These　premises　clearly　give　the　conclusion　that　there　is　in　nature　some　bodily　substance　other　than　the　formations　we　know；　prior　to　them　all　and　more　divine　than　they。　But　it　may　also　be　proved　as　follows。　We　may　take　it　that　all　movement　is　either　natural　or　unnatural；　and　that　the　movement　which　is　unnatural　to　one　body　is　natural　to　another…as；　for　instance；　is　the　case　with　the　upward　and　downward　movements；　which　are　natural　and　unnatural　to　fire　and　earth　respectively。　It　necessarily　follows　that　circular　movement；　being　unnatural　to　these　bodies；　is　the　natural　movement　of　some　other。　Further；　if；　on　the　one　hand；　circular　movement　is　natural　to　something；　it　must　surely　be　some　simple　and　primary　body　which　is　ordained　to　move　with　a　natural　circular　motion；　as　fire　is　ordained　to　fly　up　and　earth　down。　If；　on　the　other　hand；　the　movement　of　the　rotating　bodies　about　the　centre　is　unnatural；　it　would　be　remarkable　and　indeed　quite　inconceivable　that　this　movement　alone　should　be　continuous　and　eternal；　being　nevertheless　contrary　to　nature。　At　any　rate　the　evidence　of　all　other　cases　goes　to　show　that　it　is　the　unnatural　which　quickest　passes　away。　And　so；　if；　as　some　say；　the　body　so　moved　is　fire；　this　movement　is　just　as　unnatural　to　it　as　downward　movement；　for　any　one　can　see　that　fire　moves　in　a　straight　line　away　from　the　centre。　On　all　these　grounds；　therefore；　we　may　infer　with　confidence　that　there　is　something　beyond　the　bodies　that　are　about　us　on　this　earth；　different　and　separate　from　them；　and　that　the　superior　glory　of　its　nature　is　proportionate　to　its　distance　from　this　world　of　ours。
  3
  In　consequence　of　what　has　been　said；　in　part　by　way　of　assumption　and　in　part　by　way　of　proof；　it　is　clear　that　not　every　body　either　possesses　lightness　or　heaviness。　As　a　preliminary　we　must　explain　in　what　sense　we　are　using　the　words　'heavy'　and　'light'；　sufficiently；　at　least；　for　our　present　purpose：　we　can　examine　the　terms　more　closely　later；　when　we　come　to　consider　their　essential　nature。　Let　us　then　apply　the　term　'heavy'　to　that　which　naturally　moves　towards　the　c