。　The　reason　is　that　there　is　more　void。　And　this　would　also　account　for　the　fact　that　a　body　composed　of　a　number　of　solid　parts　equal　to；　or　even　smaller　than；　that　of　another　is　sometimes　larger　in　bulk　than　it。　In　short；　generally　and　in　every　case　a　body　is　relatively　light　when　it　contains　a　relatively　large　amount　of　void。　This　is　the　way　they　put　it　themselves；　but　their　account　requires　an　addition。　Relative　lightness　must　depend　not　only　on　an　excess　of　void；　but　also　an　a　defect　of　solid：　for　if　the　ratio　of　solid　to　void　exceeds　a　certain　proportion；　the　relative　lightness　will　disappear。　Thus　fire；　they　say；　is　the　lightest　of　things　just　for　this　reason　that　it　has　the　most　void。　But　it　would　follow　that　a　large　mass　of　gold；　as　containing　more　void　than　a　small　mass　of　fire；　is　lighter　than　it；　unless　it　also　contains　many　times　as　much　solid。　The　addition　is　therefore　necessary。　　　Of　those　who　deny　the　existence　of　a　void　some；　like　Anaxagoras　and　Empedocles；　have　not　tried　to　analyse　the　notions　of　light　and　heavy　at　all；　and　those　who；　while　still　denying　the　existence　of　a　void；　have　attempted　this；　have　failed　to　explain　why　there　are　bodies　which　are　absolutely　heavy　and　light；　or　in　other　words　why　some　move　upward　and　others　downward。　The　fact；　again；　that　the　body　of　greater　bulk　is　sometimes　lighter　than　smaller　bodies　is　one　which　they　have　passed　over　in　silence；　and　what　they　have　said　gives　no　obvious　suggestion　for　reconciling　their　views　with　the　observed　facts。　　　But　those　who　attribute　the　lightness　of　fire　to　its　containing　so　much　void　are　necessarily　involved　in　practically　the　same　difficulties。　For　though　fire　be　supposed　to　contain　less　solid　than　any　other　body；　as　well　as　more　void；　yet　there　will　be　a　certain　quantum　of　fire　in　which　the　amount　of　solid　or　plenum　is　in　excess　of　the　solids　contained　in　some　small　quantity　of　earth。　They　may　reply　that　there　is　an　excess　of　void　also。　But　the　question　is；　how　will　they　discriminate　the　absolutely　heavy？　Presumably；　either　by　its　excess　of　solid　or　by　its　defect　of　void。　On　the　former　view　there　could　be　an　amount　of　earth　so　small　as　to　contain　less　solid　than　a　large　mass　of　fire。　And　similarly；　if　the　distinction　rests　on　the　amount　of　void；　there　will　be　a　body；　lighter　than　the　absolutely　light；　which　nevertheless　moves　downward　as　constantly　as　the　other　moves　upward。　But　that　cannot　be　so；　since　the　absolutely　light　is　always　lighter　than　bodies　which　have　weight　and　move　downward；　while；　on　the　other　hand；　that　which　is　lighter　need　not　be　light；　because　in　common　speech　we　distinguish　a　lighter　and　a　heavier　（viz。　water　and　earth）　among　bodies　endowed　with　weight。　Again；　the　suggestion　of　a　certain　ratio　between　the　void　and　the　solid　in　a　body　is　no　more　equal　to　solving　the　problem　before　us。　The　manner　of　speaking　will　issue　in　a　similar　impossibility。　For　any　two　portions　of　fire；　small　or　great；　will　exhibit　the　same　ratio　of　solid　to　void；　but　the　upward　movement　of　the　greater　is　quicker　than　that　of　the　less；　just　as　the　downward　movement　of　a　mass　of　gold　or　lead；　or　of　any　other　body　endowed　with　weight；　is　quicker　in　proportion　to　its　size。　This；　however；　should　not　be　the　case　if　the　ratio　is　the　ground　of　distinction　between　heavy　things　and　light。　There　is　also　an　absurdity　in　attributing　the　upward　movement　of　bodies　to　a　void　which　does　not　itself　move。　If；　however；　it　is　the　nature　of　a　void　to　move　upward　and　of　a　plenum　to　move　downward；　and　therefore　each　causes　a　like　movement　in　other　things；　there　was　no　need　to　raise　the　question　why　composite　bodies　are　some　light　and　some　heavy；　they　had　only　to　explain　why　these　two　things　are　themselves　light　and　heavy　respectively；　and　to　give；　further；　the　reason　why　the　plenum　and　the　void　are　not　eternally　separated。　It　is　also　unreasonable　to　imagine　a　place　for　the　void；　as　if　the　void　were　not　itself　a　kind　of　place。　But　if　the　void　is　to　move；　it　must　have　a　place　out　of　which　and　into　which　the　change　carries　it。　Also　what　is　the　cause　of　its　movement？　Not；　surely；　its　voidness：　for　it　is　not　the　void　only　which　is　moved；　but　also　the　solid。　　　Similar　difficulties　are　involved　in　all　other　methods　of　distinction；　whether　they　account　for　the　relative　lightness　and　heaviness　of　bodies　by　distinctions　of　size；　or　proceed　on　any　other　principle；　so　long　as　they　attribute　to　each　the　same　matter；　or　even　if　they　recognize　more　than　one　matter；　so　long　as　that　means　only　a　pair　of　contraries。　If　there　is　a　single　matter；　as　with　those　who　compose　things　of　triangles；　nothing　can　be　absolutely　heavy　or　light：　and　if　there　is　one　matter　and　its　contrary…the　void；　for　instance；　and　the　plenum…no　reason　can　be　given　for　the　relative　lightness　and　heaviness　of　the　bodies　intermediate　between　the　absolutely　light　and　heavy　when　compared　either　with　one　another　or　with　these　themselves。　The　view　which　bases　the　distinction　upon　differences　of　size　is　more　like　a　mere　fiction　than　those　previously　mentioned；　but；　in　that　it　is　able　to　make　distinctions　between　the　four　elements；　it　is　in　a　stronger　position　for　meeting　the　foregoing　difficulties。　Since；　however；　it　imagines　that　these　bodies　which　differ　in　size　are　all　made　of　one　substance；　it　implies；　equally　with　the　view　that　there　is　but　one　matter；　that　there　is　nothing　absolutely　light　and　nothing　which　moves　upward　（except　as　being　passed　by　other　things　or　forced　up　by　them）；　and　since　a　multitude　of　small　atoms　are　heavier　than　a　few　large　ones；　it　will　follow　that　much　air　or　fire　is　heavier　than　a　little　water　or　earth；　which　is　impossible。
  3
  These；　then；　are　the　views　which　have　been　advanced　by　others　and　the　terms　in　which　they　state　them。　We　may　begin　our　own　statement　by　settling　a　question　which　to　some　has　been　the　main　difficulty…the　question　why　some　bodies　move　always　and　naturally　upward　and　others　downward；　while　others　again　move　both　upward　and　downward。　After　that　we　will　inquire　into　light　and　heavy　and　of　the　various　phenomena　connected　with　them。　The　local　movement　of　each　body　into　its　own　place　must　be　regarded　as　similar　to　what　happens　in　connexion　with　other　forms　of　generation　and　change。　There　are；　in　fact；　three　kinds　of　movement；　affecting　respectively　the　size；　the　form；　and　the　place　of　a　thing；　and　in　each　it　is　observable　that　change　proceeds　from　a　contrary　to　a　contrary　or　to　something　intermediate：　it　is　never　the　change　of　any　chance　subject　in　any　chance　direction；　nor；　similarly；　is　the　relation　of　the　mover　to　its　object　fortuitous：　the　thing　altered　is　different　from　the　thing　increased；　and　precisely　the　same　difference　holds　between　that　which　produces　alteration　and　that　which　produces　increase。　In　the　same　manner　it　must　be　thought　that　produces　local　motion　and　that　which　is　so　moved　are　not　fortuitously　related。　Now；　that　which　produces　upward　and　downward　movement　is　that　which　produces　weight　and　lightness；　and　that　which　is　moved　is　that　which　is　potentially　heavy　or　light；　and　the　movement　of　each　body　to　its　own　place　is　motion　towards　its　own　form。　（It　is　best　to　interpret　in　this　sense　the　common　statement　of　the　older　writers　that　'like　moves　to　like'。　For　the　words　are　not　in　every　sense　true　to　fact。　If　one　were　to　remove　the　earth　to　where　the　moon　now　is；　the　various　fragments　of　earth　would　each　move　not　towards　it　but　to　the　place　in　which　it　now　is。　In　general；　when　a　number　of　similar　and　undifferentiated　bodies　are　moved　with　the　same　motion　this　result　is　necessarily　produced；　viz。　that　the　place　which　is　the　natural　goal　of　the　movement　of　each　single　part　is　also　that　of　the　whole。　But　since　the　place　of　a　thing　is　the　boundary　of　that　which　contains　it；　and　the　continent　of　all　things　that　move　upward　or　downward　is　the　extremity　and　the　centre；　and　this　boundary　comes　to　be；　in　a　sense；　the　form　of　that　which　is　contained；　it　is　to　its　like　that　a　body　moves　when　it　moves　to　its　own　place。　For　the　successive　members　of　the　scries　are　like　one　another：　water；　I　mean；　is　like　air　and　air　like　fire；　and　between　intermediates　the　relation　may　be　converted；　though　not　between　them　and　the　extremes；　thus　air　is　like　water；　but　water　is　like　earth：　for　the　relation　of　each　outer　body　to　that　which　is　next　within　it　is　that　of　form　to　matter。）　Thus　to　ask　why　fire　moves　upward　and　earth　downward　is　the　same　as　to　ask　why　the　healable；　when　moved　and　changed　qua　healable；　attains　health　and　not　whiteness；　and　similar　questions　might　be　asked　concerning　any　other　subject　of　aletion。　Of　course　the　subject　of　increase；　when　changed　qua　increasable；　attains　not　health　but　a　superior　size。　The　same　applies　in　the　other　cases。　One　thing　changes　in　quality；　another　in　quantity：　and　so　in　place；　a　light　thing　goes　upward；　a　heavy　thing　downward。　The　only　difference　is　that　in　the　last　case；　viz。　that　of　the　heavy　and　the　light；　the　bodies　are　thought　to　have　a　spring　of　change　within　themselves；　while　the　subjects　of　healing　and　increase　are　thought　to　be　moved　purely　from　without。　Sometimes；　however；　even　they　change　of　themselves；　ie。　in　response　to　a　slight　external　movement　reach　health　or　increase；　as　the　case　may　be。　And　sinc