potency　must　be　two　in　action；　and　so　be　a　definite　spatial
  magnitude　not　a　mathematical　point。　Again；　C　may　be　moved
  simultaneously　with　B。　Both　the　originals　then　in　A　must　move　and
  be；　and　so　there　must　be　something　other　than　them　which　moves　but
  is　not　moved。　For　otherwise；　when　the　movement　begins；　the　extremes；
  i。e。　the　originals；　in　A　would　rest　upon　one　another；　like　two　men
  putting　themselves　back　to　back　and　so　moving　their　legs。　There　must
  then　be　some　one　thing　which　moves　both。　This　something　is　the　soul；
  distinct　from　the　spatial　magnitude　just　described　and　yet　located
  therein。
  10
  Although　from　the　point　of　view　of　the　definition　of　movement…　a
  definition　which　gives　the　cause…　desire　is　the　middle　term　or
  cause；　and　desire　moves　being　moved；　still　in　the　material　animated
  body　there　must　be　some　material　which　itself　moves　being　moved。　Now
  that　which　is　moved；　but　whose　nature　is　not　to　initiate　movement；
  is　capable　of　being　passive　to　an　external　force；　while　that　which
  initiates　movement　must　needs　possess　a　kind　of　force　and　power。　Now
  experience　shows　us　that　animals　do　both　possess　connatural　spirit　and
  derive　power　from　this。　（How　this　connatural　spirit　is　maintained　in
  the　body　is　explained　in　other　passages　of　our　works。）　And　this　spirit
  appears　to　stand　to　the　soul…centre　or　original　in　a　relation
  analogous　to　that　between　the　point　in　a　joint　which　moves　being　moved
  and　the　unmoved。　Now　since　this　centre　is　for　some　animals　in　the
  heart；　in　the　rest　in　a　part　analogous　with　the　heart；　we　further
  see　the　reason　for　the　connatural　spirit　being　situate　where　it
  actually　is　found。　The　question　whether　the　spirit　remains　always
  the　same　or　constantly　changes　and　is　renewed；　like　the　cognate
  question　about　the　rest　of　the　parts　of　the　body；　is　better　postponed。
  At　all　events　we　see　that　it　is　well　disposed　to　excite　movement　and
  to　exert　power；　and　the　functions　of　movement　are　thrusting　and
  pulling。　Accordingly；　the　organ　of　movement　must　be　capable　of
  expanding　and　contracting；　and　this　is　precisely　the　characteristic　of
  spirit。　It　contracts　and　expands　naturally；　and　so　is　able　to　pull　and
  to　thrust　from　one　and　the　same　cause；　exhibiting　gravity　compared
  with　the　fiery　element；　and　levity　by　comparison　with　the　opposites　of
  fire。　Now　that　which　is　to　initiate　movement　without　change　of
  structure　must　be　of　the　kind　described；　for　the　elementary　bodies
  prevail　over　one　another　in　a　compound　body　by　dint　of
  disproportion；　the　light　is　overcome　and　kept　down　by　the　heavier；　and
  the　heavy　kept　up　by　the　lighter。
  We　have　now　explained　what　the　part　is　which　is　moved　when　the
  soul　originates　movement　in　the　body；　and　what　is　the　reason　for　this。
  And　the　animal　organism　must　be　conceived　after　the　similitude　of　a
  well…governed　commonwealth。　When　order　is　once　established　in　it　there
  is　no　more　need　of　a　separate　monarch　to　preside　over　each　several
  task。　The　individuals　each　play　their　assigned　part　as　it　is
  ordered；　and　one　thing　follows　another　in　its　accustomed　order。　So
  in　animals　there　is　the　same　orderliness…　nature　taking　the　place　of
  custom…　and　each　part　naturally　doing　his　own　work　as　nature　has
  composed　them。　There　is　no　need　then　of　a　soul　in　each　part；　but　she
  resides　in　a　kind　of　central　governing　place　of　the　body；　and　the
  remaining　parts　live　by　continuity　of　natural　structure；　and　play
  the　parts　Nature　would　have　them　play。
  11
  So　much　then　for　the　voluntary　movements　of　animal　bodies；　and　the
  reasons　for　them。　These　bodies；　however；　display　in　certain　members
  involuntary　movements　too；　but　most　often　non…voluntary　movements。
  By　involuntary　I　mean　motions　of　the　heart　and　of　the　privy　member；
  for　often　upon　an　image　arising　and　without　express　mandate　of　the
  reason　these　parts　are　moved。　By　non…voluntary　I　mean　sleep　and　waking
  and　respiration；　and　other　similar　organic　movements。　For　neither
  imagination　nor　desire　is　properly　mistress　of　any　of　these；　but　since
  the　animal　body　must　undergo　natural　changes　of　quality；　and　when
  the　parts　are　so　altered　some　must　increase　and　other　decrease；　the
  body　must　straightway　be　moved　and　change　with　the　changes　that　nature
  makes　dependent　upon　one　another。　Now　the　causes　of　the　movements
  are　natural　changes　of　temperature；　both　those　coming　from　outside　the
  body；　and　those　taking　place　within　it。　So　the　involuntary　movements
  which　occur　in　spite　of　reason　in　the　aforesaid　parts　occur　when　a
  change　of　quality　supervenes。　For　conception　and　imagination；　as　we
  said　above；　produce　the　conditions　necessary　to　affections；　since　they
  bring　to　bear　the　images　or　forms　which　tend　to　create　these　states。
  And　the　two　parts　aforesaid　display　this　motion　more　conspicuously
  than　the　rest；　because　each　is　in　a　sense　a　separate　vital　organism；
  the　reason　being　that　each　contains　vital　moisture。　In　the　case　of　the
  heart　the　cause　is　plain；　for　the　heart　is　the　seat　of　the　senses；
  while　an　indication　that　the　generative　organ　too　is　vital　is　that
  there　flows　from　it　the　seminal　potency；　itself　a　kind　of　organism。
  Again；　it　is　a　reasonable　arrangement　that　the　movements　arise　in
  the　centre　upon　movements　in　the　parts；　and　in　the　parts　upon
  movements　in　the　centre；　and　so　reach　one　another。　Conceive　A　to　be
  the　centre　or　starting　point。　The　movements　then　arrive　at　the
  centre　from　each　letter　in　the　diagram　we　have　drawn；　and　flow　back
  again　from　the　centre　which　is　moved　and　changes；　（for　the　centre　is
  potentially　multiple）　the　movement　of　B　goes　to　B；　that　of　C　to　C；　the
  movement　of　both　to　both；　but　from　B　to　C　the　movements　flow　by　dint
  of　going　from　B　to　A　as　to　a　centre；　and　then　from　A　to　C　as　from　a
  centre。
  Moreover　a　movement　contrary　to　reason　sometimes　does　and
  sometimes　does　not　arise　in　the　organs　on　the　occasion　of　the　same
  thoughts；　the　reason　is　that　sometimes　the　matter　which　is　passive
  to　the　impressions　is　there　in　sufficient　quantity　and　of　the　right
  quality　and　sometimes　not。
  And　so　we　have　finished　our　account　of　the　reasons　for　the　parts
  of　each　kind　of　animal；　of　the　soul；　and　furthere　of　sense…perception；
  of　sleep；　of　memory；　and　of　movement　in　general；　it　remains　to　speak
  of　animal　generation。
  …THE　END…
  。