ie。　in　response　to　a　slight　external　movement　reach　health　or　increase；　as　the　case　may　be。　And　since　the　same　thing　which　is　healable　is　also　receptive　of　disease；　it　depends　on　whether　it　is　moved　qua　healable　or　qua　liable　to　disease　whether　the　motion　is　towards　health　or　towards　disease。　But　the　reason　why　the　heavy　and　the　light　appear　more　than　these　things　to　contain　within　themselves　the　source　of　their　movements　is　that　their　matter　is　nearest　to　being。　This　is　indicated　by　the　fact　that　locomotion　belongs　to　bodies　only　when　isolated　from　other　bodies；　and　is　generated　last　of　the　several　kinds　of　movement；　in　order　of　being　then　it　will　be　first。　Now　whenever　air　comes　into　being　out　of　water；　light　out　of　heavy；　it　goes　to　the　upper　place。　It　is　forthwith　light：　becoming　is　at　an　end；　and　in　that　place　it　has　being。　Obviously；　then；　it　is　a　potentiality；　which；　in　its　passage　to　actuality；　comes　into　that　place　and　quantity　and　quality　which　belong　to　its　actuality。　And　the　same　fact　explains　why　what　is　already　actually　fire　or　earth　moves；　when　nothing　obstructs　it；　towards　its　own　place。　For　motion　is　equally　immediate　in　the　case　of　nutriment；　when　nothing　hinders；　and　in　the　case　of　the　thing　healed；　when　nothing　stays　the　healing。　But　the　movement　is　also　due　to　the　original　creative　force　and　to　that　which　removes　the　hindrance　or　off　which　the　moving　thing　rebounded；　as　was　explained　in　our　opening　discussions；　where　we　tried　to　show　how　none　of　these　things　moves　itself。　The　reason　of　the　various　motions　of　the　various　bodies；　and　the　meaning　of　the　motion　of　a　body　to　its　own　place；　have　now　been　explained。
  4
  We　have　now　to　speak　of　the　distinctive　properties　of　these　bodies　and　of　the　various　phenomena　connected　with　them。　In　accordance　with　general　conviction　we　may　distinguish　the　absolutely　heavy；　as　that　which　sinks　to　the　bottom　of　all　things；　from　the　absolutely　light；　which　is　that　which　rises　to　the　surface　of　all　things。　I　use　the　term　'absolutely'；　in　view　of　the　generic　character　of　'light'　and　'heavy'；　in　order　to　confine　the　application　to　bodies　which　do　not　combine　lightness　and　heaviness。　It　is　apparent；　I　mean；　that　fire；　in　whatever　quantity；　so　long　as　there　is　no　external　obstacle　moves　upward；　and　earth　downward；　and；　if　the　quantity　is　increased；　the　movement　is　the　same；　though　swifter。　But　the　heaviness　and　lightness　of　bodies　which　combine　these　qualities　is　different　from　this；　since　while　they　rise　to　the　surface　of　some　bodies　they　sink　to　the　bottom　of　others。　Such　are　air　and　water。　Neither　of　them　is　absolutely　either　light　or　heavy。　Both　are　lighter　than　earth…for　any　portion　of　either　rises　to　the　surface　of　it…but　heavier　than　fire；　since　a　portion　of　either；　whatever　its　quantity；　sinks　to　the　bottom　of　fire；　compared　together；　however；　the　one　has　absolute　weight；　the　other　absolute　lightness；　since　air　in　any　quantity　rises　to　the　surface　of　water；　while　water　in　any　quantity　sinks　to　the　bottom　of　air。　Now　other　bodies　are　severally　light　and　heavy；　and　evidently　in　them　the　attributes　are　due　to　the　difference　of　their　uncompounded　parts：　that　is　to　say；　according　as　the　one　or　the　other　happens　to　preponderate　the　bodies　will　be　heavy　and　light　respectively。　Therefore　we　need　only　speak　of　these　parts；　since　they　are　primary　and　all　else　consequential：　and　in　so　doing　we　shall　be　following　the　advice　which　we　gave　to　those　whose　attribute　heaviness　to　the　presence　of　plenum　and　lightness　to　that　of　void。　It　is　due　to　the　properties　of　the　elementary　bodies　that　a　body　which　is　regarded　as　light　in　one　place　is　regarded　as　heavy　in　another；　and　vice　versa。　In　air；　for　instance；　a　talent's　weight　of　wood　is　heavier　than　a　mina　of　lead；　but　in　water　the　wood　is　the　lighter。　The　reason　is　that　all　the　elements　except　fire　have　weight　and　all　but　earth　lightness。　Earth；　then；　and　bodies　in　which　earth　preponderates；　must　needs　have　weight　everywhere；　while　water　is　heavy　anywhere　but　in　earth；　and　air　is　heavy　when　not　in　water　or　earth。　In　its　own　place　each　of　these　bodies　has　weight　except　fire；　even　air。　Of　this　we　have　evidence　in　the　fact　that　a　bladder　when　inflated　weighs　more　than　when　empty。　A　body；　then；　in　which　air　preponderates　over　earth　and　water；　may　well　be　lighter　than　something　in　water　and　yet　heavier　than　it　in　air；　since　such　a　body　does　not　rise　in　air　but　rises　to　the　surface　in　water。　　　The　following　account　will　make　it　plain　that　there　is　an　absolutely　light　and　an　absolutely　heavy　body。　And　by　absolutely　light　I　mean　one　which　of　its　own　nature　always　moves　upward；　by　absolutely　heavy　one　which　of　its　own　nature　always　moves　downward；　if　no　obstacle　is　in　the　way。　There　are；　I　say；　these　two　kinds　of　body；　and　it　is　not　the　case；　as　some　maintain；　that　all　bodies　have　weight。　Different　views　are　in　fact　agreed　that　there　is　a　heavy　body；　which　moves　uniformly　towards　the　centre。　But　is　also　similarly　a　light　body。　For　we　see　with　our　eyes；　as　we　said　before；　that　earthy　things　sink　to　the　bottom　of　all　things　and　move　towards　the　centre。　But　the　centre　is　a　fixed　point。　If　therefore　there　is　some　body　which　rises　to　the　surface　of　all　things…and　we　observe　fire　to　move　upward　even　in　air　itself；　while　the　air　remains　at　rest…clearly　this　body　is　moving　towards　the　extremity。　It　cannot　then　have　any　weight。　If　it　had；　there　would　be　another　body　in　which　it　sank：　and　if　that　had　weight；　there　would　be　yet　another　which　moved　to　the　extremity　and　thus　rose　to　the　surface　of　all　moving　things。　In　fact；　however；　we　have　no　evidence　of　such　a　body。　Fire；　then；　has　no　weight。　Neither　has　earth　any　lightness；　since　it　sinks　to　the　bottom　of　all　things；　and　that　which　sinks　moves　to　the　centre。　That　there　is　a　centre　towards　which　the　motion　of　heavy　things；　and　away　from　which　that　of　light　things　is　directed；　is　manifest　in　many　ways。　First；　because　no　movement　can　continue　to　infinity。　For　what　cannot　be　can　no　more　come…to…be　than　be；　and　movement　is　a　coming　to…be　in　one　place　from　another。　Secondly；　like　the　upward　movement　of　fire；　the　downward　movement　of　earth　and　all　heavy　things　makes　equal　angles　on　every　side　with　the　earth's　surface：　it　must　therefore　be　directed　towards　the　centre。　Whether　it　is　really　the　centre　of　the　earth　and　not　rather　that　of　the　whole　to　which　it　moves；　may　be　left　to　another　inquiry；　since　these　are　coincident。　But　since　that　which　sinks　to　the　bottom　of　all　things　moves　to　the　centre；　necessarily　that　which　rises　to　the　surface　moves　to　the　extremity　of　the　region　in　which　the　movement　of　these　bodies　takes　place。　For　the　centre　is　opposed　as　contrary　to　the　extremity；　as　that　which　sinks　is　opposed　to　that　which　rises　to　the　surface。　This　also　gives　a　reasonable　ground　for　the　duality　of　heavy　and　light　in　the　spatial　duality　centre　and　extremity。　Now　there　is　also　the　intermediate　region　to　which　each　name　is　given　in　opposition　to　the　other　extreme。　For　that　which　is　intermediate　between　the　two　is　in　a　sense　both　extremity　and　centre。　For　this　reason　there　is　another　heavy　and　light；　namely；　water　and　air。　But　in　our　view　the　continent　pertains　to　form　and　the　contained　to　matter：　and　this　distinction　is　present　in　every　genus。　Alike　in　the　sphere　of　quality　and　in　that　of　quantity　there　is　that　which　corresponds　rather　to　form　and　that　which　corresponds　to　matter。　In　the　same　way；　among　spatial　distinctions；　the　above　belongs　to　the　determinate；　the　below　to　matter。　The　same　holds；　consequently；　also　of　the　matter　itself　of　that　which　is　heavy　and　light：　as　potentially　possessing　the　one　character；　it　is　matter　for　the　heavy；　and　as　potentially　possessing　the　other；　for　the　light。　It　is　the　same　matter；　but　its　being　is　different；　as　that　which　is　receptive　of　disease　is　the　same　as　that　which　is　receptive　of　health；　though　in　being　different　from　it；　and　therefore　diseasedness　is　different　from　healthiness。
  5
  A　thing　then　which　has　the　one　kind　of　matter　is　light　and　always　moves　upward；　while　a　thing　which　has　the　opposite　matter　is　heavy　and　always　moves　downward。　Bodies　composed　of　kinds　of　matter　different　from　these　but　having　relatively　to　each　other　the　character　which　these　have　absolutely；　possess　both　the　upward　and　the　downward　motion。　Hence　air　and　water　each　have　both　lightness　and　weight；　and　water　sinks　to　the　bottom　of　all　things　except　earth；　while　air　rises　to　the　surface　of　all　things　except　fire。　But　since　there　is　one　body　only　which　rises　to　the　surface　of　all　things　and　one　only　which　sinks　to　the　bottom　of　all　things；　there　must　needs　be　two　other　bodies　which　sink　in　some　bodies　and　rise　to　the　surface　of　others。　The　kinds　of　matter；　then；　must　be　as　numerous　as　these　bodies；　i。e。　four；　but　though　they　are　four　there　must　be　a　common　matter　of　all…particularly　if　they　pass　into　one　another…which　in　each　is　in　being　different。　There　is　no　reason　why　there　should　not　be　one　or　more　intermediates　between　the　contraries；　as　in　the　case　of　colour；　for　'intermediate'　and　'mean'　are　capable　of　more　than　one　application。　　　Now　in　its　own　place　every　body　endowed　with　both　weight　and　lightness　has　weightwhereas　earth