ON　THE　MOTION　OF　ANIMALS
  by　Aristotle
  translated　by　A。　S。　L。　Farquharson
  1
  ELSEWHERE　we　have　investigated　in　detail　the　movement　of　animals
  after　their　various　kinds；　the　differences　between　them；　and　the
  reasons　for　their　particular　characters　（for　some　animals　fly；　some
  swim；　some　walk；　others　move　in　various　other　ways）；　there　remains
  an　investigation　of　the　common　ground　of　any　sort　of　animal　movement
  whatsoever。
  Now　we　have　already　determined　（when　we　were　discussing　whether
  eternal　motion　exists　or　not；　and　its　definition；　if　it　does　exist）
  that　the　origin　of　all　other　motions　is　that　which　moves　itself；　and
  that　the　origin　of　this　is　the　immovable；　and　that　the　prime　mover
  must　of　necessity　be　immovable。　And　we　must　grasp　this　not　only
  generally　in　theory；　but　also　by　reference　to　individuals　in　the　world
  of　sense；　for　with　these　in　view　we　seek　general　theories；　and　with
  these　we　believe　that　general　theories　ought　to　harmonize。　Now　in
  the　world　of　sense　too　it　is　plainly　impossible　for　movement　to　be
  initiated　if　there　is　nothing　at　rest；　and　before　all　else　in　our
  present　subject…　animal　life。　For　if　one　of　the　parts　of　an　animal
  be　moved；　another　must　be　at　rest；　and　this　is　the　purpose　of　their
  joints；　animals　use　joints　like　a　centre；　and　the　whole　member；　in
  which　the　joint　is；　becomes　both　one　and　two；　both　straight　and
  bent；　changing　potentially　and　actually　by　reason　of　the　joint。　And
  when　it　is　bending　and　being　moved　one　of　the　points　in　the　joint　is
  moved　and　one　is　at　rest；　just　as　if　the　points　A　and　D　of　a
  diameter　were　at　rest；　and　B　were　moved；　and　DAC　were　generated。
  However；　in　the　geometrical　illustration；　the　centre　is　held　to　be
  altogether　indivisible　（for　in　mathematics　motion　is　a　fiction；　as　the
  phrase　goes；　no　mathematical　entity　being　really　moved）；　whereas　in
  the　case　of　joints　the　centres　become　now　one　potentially　and
  divided　actually；　and　now　one　actually　and　divided　potentially。　But
  still　the　origin　of　movement；　qua　origin；　always　remains　at　rest
  when　the　lower　part　of　a　limb　is　moved；　for　example；　the　elbow
  joint；　when　the　forearm　is　moved；　and　the　shoulder；　when　the　whole
  arm；　the　knee　when　the　tibia　is　moved；　and　the　hip　when　the　whole　leg。
  Accordingly　it　is　plain　that　each　animal　as　a　whole　must　have　within
  itself　a　point　at　rest；　whence　will　be　the　origin　of　that　which　is
  moved；　and　supporting　itself　upon　which　it　will　be　moved　both　as　a
  complete　whole　and　in　its　members。
  2
  But　the　point　of　rest　in　the　animal　is　still　quite　ineffectual
  unless　there　be　something　without　which　is　absolutely　at　rest　and
  immovable。　Now　it　is　worth　while　to　pause　and　consider　what　has　been
  said；　for　it　involves　a　speculation　which　extends　beyond　animals
  even　to　the　motion　and　march　of　the　universe。　For　just　as　there　must
  be　something　immovable　within　the　animal；　if　it　is　to　be　moved；　so
  even　more　must　there　be　without　it　something　immovable；　by
  supporting　itself　upon　which　that　which　is　moved　moves。　For　were
  that　something　always　to　give　way　（as　it　does　for　mice　walking　in
  grain　or　persons　walking　in　sand）　advance　would　be　impossible；　and
  neither　would　there　be　any　walking　unless　the　ground　were　to　remain
  still；　nor　any　flying　or　swimming　were　not　the　air　and　the　sea　to
  resist。　And　this　which　resists　must　needs　be　different　from　what　is
  moved；　the　whole　of　it　from　the　whole　of　that；　and　what　is　thus
  immovable　must　be　no　part　of　what　is　moved；　otherwise　there　will　be　no
  movement。　Evidence　of　this　lies　in　the　problem　why　it　is　that　a　man
  easily　moves　a　boat　from　outside；　if　he　push　with　a　pole；　putting　it
  against　the　mast　or　some　other　part；　but　if　he　tried　to　do　this　when
  in　the　boat　itself　he　would　never　move　it；　no　not　giant　Tityus　himself
  nor　Boreas　blowing　from　inside　the　ship；　if　he　really　were　blowing
  in　the　way　painters　represent　him；　for　they　paint　him　sending　the
  breath　out　from　the　boat。　For　whether　one　blew　gently　or　so　stoutly　as
  to　make　a　very　great　wind；　and　whether　what　were　thrown　or　pushed　were
  wind　or　something　else；　it　is　necessary　in　the　first　place　to　be
  supported　upon　one　of　one's　own　members　which　is　at　rest　and　so　to
  push；　and　in　the　second　place　for　this　member；　either　itself；　or
  that　of　which　it　is　a　part；　to　remain　at　rest；　fixing　itself　against
  something　external　to　itself。　Now　the　man　who　is　himself　in　the
  boat；　if　he　pushes；　fixing　himself　against　the　boat；　very　naturally
  does　not　move　the　boat；　because　what　he　pushes　against　should　properly
  remain　at　rest。　Now　what　he　is　trying　to　move；　and　what　he　is　fixing
  himself　against　is　in　his　case　the　same。　If；　however；　he　pushes　or
  pulls　from　outside　he　does　move　it；　for　the　ground　is　no　part　of　the
  boat。
  3
  Here　we　may　ask　the　difficult　question　whether　if　something　moves
  the　whole　heavens　this　mover　must　be　immovable；　and　moreover　be　no
  part　of　the　heavens；　nor　in　the　heavens。　For　either　it　is　moved　itself
  and　moves　the　heavens；　in　which　case　it　must　touch　something　immovable
  in　order　to　create　movement；　and　then　this　is　no　part　of　that　which
  creates　movement；　or　if　the　mover　is　from　the　first　immovable　it
  will　equally　be　no　part　of　that　which　is　moved。　In　this　point　at　least
  they　argue　correctly　who　say　that　as　the　Sphere　is　carried　round　in
  a　circle　no　single　part　remains　still；　for　then　either　the　whole　would
  necessarily　stand　still　or　its　continuity　be　torn　asunder；　but　they
  argue　less　well　in　supposing　that　the　poles　have　a　certain　force；
  though　conceived　as　having　no　magnitude；　but　as　merely　termini　or
  points。　For　besides　the　fact　that　no　such　things　have　any
  substantial　existence　it　is　impossible　for　a　single　movement　to　be
  initiated　by　what　is　twofold；　and　yet　they　make　the　poles　two。　From
  a　review　of　these　difficulties　we　may　conclude　that　there　is　something
  so　related　to　the　whole　of　Nature；　as　the　earth　is　to　animals　and
  things　moved　by　them。
  And　the　mythologists　with　their　fable　of　Atlas　setting　his　feet　upon
  the　earth　appear　to　have　based　the　fable　upon　intelligent　grounds。
  They　make　Atlas　a　kind　of　diameter　twirling　the　heavens　about　the
  poles。　Now　as　the　earth　remains　still　this　would　be　reasonable　enough；
  but　their　theory　involves　them　in　the　position　that　the　earth　is　no
  part　of　the　universe。　And　further　the　force　of　that　which　initiates
  movement　must　be　made　equal　to　the　force　of　that　which　remains　at
  rest。　For　there　is　a　definite　quantity　of　force　or　power　by　dint　of
  which　that　which　remains　at　rest　does　so；　just　as　there　is　of　force　by
  dint　of　which　that　which　initiates　movement　does　so；　and　as　there　is　a
  necessary　proportion　between　opposite　motions；　so　there　is　between
  absences　of　motion。　Now　equal　forces　are　unaffected　by　one　another；
  but　are　overcome　by　a　superiority　of　force。　And　so　in　their　theory
  Atlas；　or　whatever　similar　power　initiates　movement　from　within；
  must　exert　no　more　force　than　will　exactly　balance　the　stability　of
  the　earth…　otherwise　the　earth　will　be　moved　out　of　her　place　in　the
  centre　of　things。　For　as　the　pusher　pushes　so　is　the　pushed　pushed；
  and　with　equal　force。　But　the　prime　mover　moves　that　which　is　to　begin
  with　at　rest；　so　that　the　power　it　exerts　is　greater；　rather　than
  equal　and　like　to　the　power　which　produces　absence　of　motion　in　that
  which　is　moved。　And　similarly　also　the　power　of　what　is　moved　and　so
  moves　must　be　greater　than　the　power　of　that　which　is　moved　but　does
  not　initiate　movement。　Therefore　the　force　of　the　earth　in　its
  immobility　will　have　to　be　as　great　as　the　force　of　the　whole　heavens；
  and　of　that　which　moves　the　heavens。　But　if　that　is　impossible；　it
  follows　that　the　heavens　cannot　possibly　be　moved　by　any　force　of　this
  kind　inside　them。
  4
  There　is　a　further　difficulty　about　the　motions　of　the　parts　of
  the　heavens　which；　as　akin　to　what　has　gone　before；　may　be
  considered　next。　For　if　one　could　overcome　by　force　of　motion　the
  immobility　of　the　earth　he　would　clearly　move　it　away　from　the　centre。
  And　it　is　plain　that　the　power　from　which　this　force　would　originate
  will　not　be　infinite；　for　the　earth　is　not　infinite　and　therefore
  its　weight　is　not。　Now　there　are　more　senses　than　one　of　the　word
  'impossible'。　When　we　say　it　is　impossible　to　see　a　sound；　and　when　we
  say　it　is　impossible　to　see　the　men　in　the　moon；　we　use　two　senses
  of　the　word；　the　former　is　of　necessity；　the　latter；　though　their
  nature　is　to　be　seen；　cannot　as　a　fact　be　seen　by　us。　Now　we　suppose
  that