attribute　and　we　ask　whether　the　thing　is　thus　or　otherwise
  qualified…whether；　e。g。　the　sun　suffers　eclipse　or　not…then　we　are
  asking　as　to　the　fact　of　a　connexion。　That　our　inquiry　ceases　with　the
  discovery　that　the　sun　does　suffer　eclipse　is　an　indication　of　this；
  and　if　we　know　from　the　start　that　the　sun　suffers　eclipse；　we　do
  not　inquire　whether　it　does　so　or　not。　On　the　other　hand；　when　we　know
  the　fact　we　ask　the　reason；　as；　for　example；　when　we　know　that　the　sun
  is　being　eclipsed　and　that　an　earthquake　is　in　progress；　it　is　the
  reason　of　eclipse　or　earthquake　into　which　we　inquire。
  Where　a　complex　is　concerned；　then；　those　are　the　two　questions　we
  ask；　but　for　some　objects　of　inquiry　we　have　a　different　kind　of
  question　to　ask；　such　as　whether　there　is　or　is　not　a　centaur　or　a
  God。　（By　'is　or　is　not'　I　mean　'is　or　is　not；　without　further
  qualification'；　as　opposed　to　'is　or　is　not　＇e。g。＇　white'。）　On　the
  other　hand；　when　we　have　ascertained　the　thing's　existence；　we　inquire
  as　to　its　nature；　asking；　for　instance；　'what；　then；　is　God？'　or　'what
  is　man？'。
  2
  These；　then；　are　the　four　kinds　of　question　we　ask；　and　it　is　in　the
  answers　to　these　questions　that　our　knowledge　consists。
  Now　when　we　ask　whether　a　connexion　is　a　fact；　or　whether　a　thing
  without　qualification　is；　we　are　really　asking　whether　the　connexion
  or　the　thing　has　a　'middle'；　and　when　we　have　ascertained　either
  that　the　connexion　is　a　fact　or　that　the　thing　is…i。e。　ascertained
  either　the　partial　or　the　unqualified　being　of　the　thing…and　are
  proceeding　to　ask　the　reason　of　the　connexion　or　the　nature　of　the
  thing；　then　we　are　asking　what　the　'middle'　is。
  （By　distinguishing　the　fact　of　the　connexion　and　the　existence　of
  the　thing　as　respectively　the　partial　and　the　unqualified　being　of　the
  thing；　I　mean　that　if　we　ask　'does　the　moon　suffer　eclipse？'；　or　'does
  the　moon　wax？'；　the　question　concerns　a　part　of　the　thing's　being；　for
  what　we　are　asking　in　such　questions　is　whether　a　thing　is　this　or
  that；　i。e。　has　or　has　not　this　or　that　attribute：　whereas；　if　we　ask
  whether　the　moon　or　night　exists；　the　question　concerns　the
  unqualified　being　of　a　thing。）
  We　conclude　that　in　all　our　inquiries　we　are　asking　either　whether
  there　is　a　'middle'　or　what　the　'middle'　is：　for　the　'middle'　here
  is　precisely　the　cause；　and　it　is　the　cause　that　we　seek　in　all　our
  inquiries。　Thus；　'Does　the　moon　suffer　eclipse？'　means　'Is　there　or　is
  there　not　a　cause　producing　eclipse　of　the　moon？'；　and　when　we　have
  learnt　that　there　is；　our　next　question　is；　'What；　then；　is　this
  cause？　for　the　cause　through　which　a　thing　is…not　is　this　or　that；
  i。e。　has　this　or　that　attribute；　but　without　qualification　is…and
  the　cause　through　which　it　is…not　is　without　qualification；　but　is
  this　or　that　as　having　some　essential　attribute　or　some　accident…are
  both　alike　the　middle'。　By　that　which　is　without　qualification　I
  mean　the　subject；　e。g。　moon　or　earth　or　sun　or　triangle；　by　that　which
  a　subject　is　（in　the　partial　sense）　I　mean　a　property；　e。g。　eclipse；
  equality　or　inequality；　interposition　or　non…interposition。　For　in　all
  these　examples　it　is　clear　that　the　nature　of　the　thing　and　the　reason
  of　the　fact　are　identical：　the　question　'What　is　eclipse？'　and　its
  answer　'The　privation　of　the　moon's　light　by　the　interposition　of
  the　earth'　are　identical　with　the　question　'What　is　the　reason　of
  eclipse？'　or　'Why　does　the　moon　suffer　eclipse？'　and　the　reply
  'Because　of　the　failure　of　light　through　the　earth's　shutting　it　out'。
  Again；　for　'What　is　a　concord？　A　commensurate　numerical　ratio　of　a
  high　and　a　low　note'；　we　may　substitute　'What　ratio　makes　a　high　and　a
  low　note　concordant？　Their　relation　according　to　a　commensurate
  numerical　ratio。'　'Are　the　high　and　the　low　note　concordant？'　is
  equivalent　to　'Is　their　ratio　commensurate？'；　and　when　we　find　that　it
  is　commensurate；　we　ask　'What；　then；　is　their　ratio？'。
  Cases　in　which　the　'middle'　is　sensible　show　that　the　object　of
  our　inquiry　is　always　the　'middle'：　we　inquire；　because　we　have　not
  perceived　it；　whether　there　is　or　is　not　a　'middle'　causing；　e。g。　an
  eclipse。　On　the　other　hand；　if　we　were　on　the　moon　we　should　not　be
  inquiring　either　as　to　the　fact　or　the　reason；　but　both　fact　and
  reason　would　be　obvious　simultaneously。　For　the　act　of　perception
  would　have　enabled　us　to　know　the　universal　too；　since；　the　present
  fact　of　an　eclipse　being　evident；　perception　would　then　at　the　same
  time　give　us　the　present　fact　of　the　earth's　screening　the　sun's
  light；　and　from　this　would　arise　the　universal。
  Thus；　as　we　maintain；　to　know　a　thing's　nature　is　to　know　the　reason
  why　it　is；　and　this　is　equally　true　of　things　in　so　far　as　they　are
  said　without　qualification　to　he　as　opposed　to　being　possessed　of　some
  attribute；　and　in　so　far　as　they　are　said　to　be　possessed　of　some
  attribute　such　as　equal　to　right　angles；　or　greater　or　less。
  3
  It　is　clear；　then；　that　all　questions　are　a　search　for　a　'middle'。
  Let　us　now　state　how　essential　nature　is　revealed　and　in　what　way　it
  can　be　reduced　to　demonstration；　what　definition　is；　and　what　things
  are　definable。　And　let　us　first　discuss　certain　difficulties　which
  these　questions　raise；　beginning　what　we　have　to　say　with　a　point　most
  intimately　connected　with　our　immediately　preceding　remarks；　namely
  the　doubt　that　might　be　felt　as　to　whether　or　not　it　is　possible　to
  know　the　same　thing　in　the　same　relation；　both　by　definition　and　by
  demonstration。　It　might；　I　mean；　be　urged　that　definition　is　held　to
  concern　essential　nature　and　is　in　every　case　universal　and
  affirmative；　whereas；　on　the　other　hand；　some　conclusions　are　negative
  and　some　are　not　universal；　e。g。　all　in　the　second　figure　are
  negative；　none　in　the　third　are　universal。　And　again；　not　even　all
  affirmative　conclusions　in　the　first　figure　are　definable；　e。g。　'every
  triangle　has　its　angles　equal　to　two　right　angles'。　An　argument
  proving　this　difference　between　demonstration　and　definition　is　that
  to　have　scientific　knowledge　of　the　demonstrable　is　identical　with
  possessing　a　demonstration　of　it：　hence　if　demonstration　of　such
  conclusions　as　these　is　possible；　there　clearly　cannot　also　be
  definition　of　them。　If　there　could；　one　might　know　such　a　conclusion
  also　in　virtue　of　its　definition　without　possessing　the
  demonstration　of　it；　for　there　is　nothing　to　stop　our　having　the　one
  without　the　other。
  Induction　too　will　sufficiently　convince　us　of　this　difference；
  for　never　yet　by　defining　anything…essential　attribute　or　accident…did
  we　get　knowledge　of　it。　Again；　if　to　define　is　to　acquire　knowledge　of
  a　substance；　at　any　rate　such　attributes　are　not　substances。
  It　is　evident；　then；　that　not　everything　demonstrable　can　be
  defined。　What　then？　Can　everything　definable　be　demonstrated；　or
  not？　There　is　one　of　our　previous　arguments　which　covers　this　too。
  Of　a　single　thing　qua　single　there　is　a　single　scientific　knowledge。
  Hence；　since　to　know　the　demonstrable　scientifically　is　to　possess　the
  demonstration　of　it；　an　impossible　consequence　will　follow：…possession
  of　its　definition　without　its　demonstration　will　give　knowledge　of　the
  demonstrable。
  Moreover；　the　basic　premisses　of　demonstrations　are　definitions；　and
  it　has　already　been　shown　that　these　will　be　found　indemonstrable；
  either　the　basic　premisses　will　be　demonstrable　and　will　depend　on
  prior　premisses；　and　the　regress　will　be　endless；　or　the　primary
  truths　will　be　indemonstrable　definitions。
  But　if　the　definable　and　the　demonstrable　are　not　wholly　the　same；
  may　they　yet　be　partially　the　same？　Or　is　that　impossible；　because
  there　can　be　no　demonstration　of　the　definable？　There　can　be　none；
  because　definition　is　of　the　essential　nature　or　being　of　something；
  and　all　demonstrations　evidently　posit　and　assume　the　essential
  nature…mathematical　demonstrations；　for　example；　the　nature　of　unity
  and　the　odd；　and　all　the　other　sciences　likewise。　Moreover；　every
  demonstration　proves　a　predicate　of　a　subject　as　attaching　or　as　not
  attaching　to　it；　but　in　definition　one　thing　is　not　predicated　of
  another；　we　do　not；　e。g。　predicate　animal　of　biped　nor　biped　of
  animal；　nor　yet　figure　of　plane…plane　not　being　figure　nor　figure
  plane。　Again；　to　prove　essential　nature　is　not　the　same　as　to　prove
  the　fact　of　a　connexion。　Now　defi