defining　condition；　and　if　this　condition　actually　exists；　we　assert
  that　A　also　actually　exists。　Or　again　we　may　ask　which　side　of　a
  contradiction　the　defining　condition　necessitates：　does　it　make　the
  angles　of　a　triangle　equal　or　not　equal　to　two　right　angles？　When　we
  have　found　the　answer；　if　the　premisses　are　immediate；　we　know　fact
  and　reason　together；　if　they　are　not　immediate；　we　know　the　fact
  without　the　reason；　as　in　the　following　example：　let　C　be　the　moon；
  A　eclipse；　B　the　fact　that　the　moon　fails　to　produce　shadows　though
  she　is　full　and　though　no　visible　body　intervenes　between　us　and
  her。　Then　if　B；　failure　to　produce　shadows　in　spite　of　the　absence
  of　an　intervening　body；　is　attributable　A　to　C；　and　eclipse；　is
  attributable　to　B；　it　is　clear　that　the　moon　is　eclipsed；　but　the
  reason　why　is　not　yet　clear；　and　we　know　that　eclipse　exists；　but　we
  do　not　know　what　its　essential　nature　is。　But　when　it　is　clear　that
  A　is　attributable　to　C　and　we　proceed　to　ask　the　reason　of　this
  fact；　we　are　inquiring　what　is　the　nature　of　B：　is　it　the　earth's
  acting　as　a　screen；　or　the　moon's　rotation　or　her　extinction？　But　B　is
  the　definition　of　the　other　term；　viz。　in　these　examples；　of　the　major
  term　A；　for　eclipse　is　constituted　by　the　earth　acting　as　a　screen。
  Thus；　（1）　'What　is　thunder？'　'The　quenching　of　fire　in　cloud'；　and　（2）
  'Why　does　it　thunder？'　'Because　fire　is　quenched　in　the　cloud'；　are
  equivalent。　Let　C　be　cloud；　A　thunder；　B　the　quenching　of　fire。　Then　B
  is　attributable　to　C；　cloud；　since　fire　is　quenched　in　it；　and　A；
  noise；　is　attributable　to　B；　and　B　is　assuredly　the　definition　of
  the　major　term　A。　If　there　be　a　further　mediating　cause　of　B；　it
  will　be　one　of　the　remaining　partial　definitions　of　A。
  We　have　stated　then　how　essential　nature　is　discovered　and　becomes
  known；　and　we　see　that；　while　there　is　no　syllogism…i。e。　no
  demonstrative　syllogism…of　essential　nature；　yet　it　is　through
  syllogism；　viz。　demonstrative　syllogism；　that　essential　nature　is
  exhibited。　So　we　conclude　that　neither　can　the　essential　nature　of
  anything　which　has　a　cause　distinct　from　itself　be　known　without
  demonstration；　nor　can　it　be　demonstrated；　and　this　is　what　we
  contended　in　our　preliminary　discussions。
  9
  Now　while　some　things　have　a　cause　distinct　from　themselves；
  others　have　not。　Hence　it　is　evident　that　there　are　essential
  natures　which　are　immediate；　that　is　are　basic　premisses；　and　of　these
  not　only　that　they　are　but　also　what　they　are　must　be　assumed　or
  revealed　in　some　other　way。　This　too　is　the　actual　procedure　of　the
  arithmetician；　who　assumes　both　the　nature　and　the　existence　of
  unit。　On　the　other　hand；　it　is　possible　（in　the　manner　explained）　to
  exhibit　through　demonstration　the　essential　nature　of　things　which
  have　a　'middle'；　i。e。　a　cause　of　their　substantial　being　other　than
  that　being　itself；　but　we　do　not　thereby　demonstrate　it。
  10
  Since　definition　is　said　to　be　the　statement　of　a　thing's　nature；
  obviously　one　kind　of　definition　will　be　a　statement　of　the　meaning　of
  the　name；　or　of　an　equivalent　nominal　formula。　A　definition　in　this
  sense　tells　you；　e。g。　the　meaning　of　the　phrase　'triangular
  character'。　When　we　are　aware　that　triangle　exists；　we　inquire　the
  reason　why　it　exists。　But　it　is　difficult　thus　to　learn　the　definition
  of　things　the　existence　of　which　we　do　not　genuinely　know…the　cause　of
  this　difficulty　being；　as　we　said　before；　that　we　only　know
  accidentally　whether　or　not　the　thing　exists。　Moreover；　a　statement
  may　be　a　unity　in　either　of　two　ways；　by　conjunction；　like　the
  Iliad；　or　because　it　exhibits　a　single　predicate　as　inhering　not
  accidentally　in　a　single　subject。
  That　then　is　one　way　of　defining　definition。　Another　kind　of
  definition　is　a　formula　exhibiting　the　cause　of　a　thing's　existence。
  Thus　the　former　signifies　without　proving；　but　the　latter　will　clearly
  be　a　quasi…demonstration　of　essential　nature；　differing　from
  demonstration　in　the　arrangement　of　its　terms。　For　there　is　a
  difference　between　stating　why　it　thunders；　and　stating　what　is　the
  essential　nature　of　thunder；　since　the　first　statement　will　be
  'Because　fire　is　quenched　in　the　clouds'；　while　the　statement　of
  what　the　nature　of　thunder　is　will　be　'The　noise　of　fire　being
  quenched　in　the　clouds'。　Thus　the　same　statement　takes　a　different
  form：　in　one　form　it　is　continuous　demonstration；　in　the　other
  definition。　Again；　thunder　can　be　defined　as　noise　in　the　clouds；
  which　is　the　conclusion　of　the　demonstration　embodying　essential
  nature。　On　the　other　hand　the　definition　of　immediates　is　an
  indemonstrable　positing　of　essential　nature。
  We　conclude　then　that　definition　is　（a）　an　indemonstrable
  statement　of　essential　nature；　or　（b）　a　syllogism　of　essential
  nature　differing　from　demonstration　in　grammatical　form；　or　（c）　the
  conclusion　of　a　demonstration　giving　essential　nature。
  Our　discussion　has　therefore　made　plain　（1）　in　what　sense　and　of
  what　things　the　essential　nature　is　demonstrable；　and　in　what　sense
  and　of　what　things　it　is　not；　（2）　what　are　the　various　meanings　of　the
  term　definition；　and　in　what　sense　and　of　what　things　it　proves　the
  essential　nature；　and　in　what　sense　and　of　what　things　it　does　not；
  （3）　what　is　the　relation　of　definition　to　demonstration；　and　how　far
  the　same　thing　is　both　definable　and　demonstrable　and　how　far　it　is
  not。
  11
  We　think　we　have　scientific　knowledge　when　we　know　the　cause；　and
  there　are　four　causes：　（1）　the　definable　form；　（2）　an　antecedent　which
  necessitates　a　consequent；　（3）　the　efficient　cause；　（4）　the　final
  cause。　Hence　each　of　these　can　be　the　middle　term　of　a　proof；　for
  （a）　though　the　inference　from　antecedent　to　necessary　consequent
  does　not　hold　if　only　one　premiss　is　assumed…two　is　the
  minimum…still　when　there　are　two　it　holds　on　condition　that　they
  have　a　single　common　middle　term。　So　it　is　from　the　assumption　of　this
  single　middle　term　that　the　conclusion　follows　necessarily。　The
  following　example　will　also　show　this。　Why　is　the　angle　in　a
  semicircle　a　right　angle？…or　from　what　assumption　does　it　follow
  that　it　is　a　right　angle？　Thus；　let　A　be　right　angle；　B　the　half　of
  two　right　angles；　C　the　angle　in　a　semicircle。　Then　B　is　the　cause
  in　virtue　of　which　A；　right　angle；　is　attributable　to　C；　the　angle
  in　a　semicircle；　since　B=A　and　the　other；　viz。　C；=B；　for　C　is　half
  of　two　right　angles。　Therefore　it　is　the　assumption　of　B；　the　half
  of　two　right　angles；　from　which　it　follows　that　A　is　attributable　to
  C；　i。e。　that　the　angle　in　a　semicircle　is　a　right　angle。　Moreover；　B
  is　identical　with　（b）　the　defining　form　of　A；　since　it　is　what　A's
  definition　signifies。　Moreover；　the　formal　cause　has　already　been
  shown　to　be　the　middle。　（c）　'Why　did　the　Athenians　become　involved
  in　the　Persian　war？'　means　'What　cause　originated　the　waging　of　war
  against　the　Athenians？'　and　the　answer　is；　'Because　they　raided　Sardis
  with　the　Eretrians'；　since　this　originated　the　war。　Let　A　be　war；　B
  unprovoked　raiding；　C　the　Athenians。　Then　B；　unprovoked　raiding；　is
  true　of　C；　the　Athenians；　and　A　is　true　of　B；　since　men　make　war　on
  the　unjust　aggressor。　So　A；　having　war　waged　upon　them；　is　true　of
  B；　the　initial　aggressors；　and　B　is　true　of　C；　the　Athenians；　who　were
  the　aggressors。　Hence　here　too　the　cause…in　this　case　the　efficient
  cause…is　the　middle　term。　（d）　This　is　no　less　true　where　the　cause
  is　the　final　cause。　E。g。　why　does　one　take　a　walk　after　supper？　For
  the　sake　of　one's　health。　Why　does　a　house　exist？　For　the　preservation
  of　one's　goods。　The　end　in　view　is　in　the　one　case　health；　in　the
  other　preservation。　To　ask　the　reason　why　one　must　walk　after　supper
  is　precisely　to　ask　to　what　end　one　must　do　it。　Let　C　be　walking　after
  supper；　B　the　non…regurgitation　of　food；　A　health。　Then　let　walking
  after　supper　possess　the　property　of　preventing　food　from　rising　to
  the　orifice　of　the　stomach；　and　let　this　condition　be　healthy；　since
  it　seems　that　B；　the　non…regurgitation　of　food；　is　attributable　to
  C；　taking　a　walk；　and　that　A；　health；　is　attributable　to　B。　What；
  then；　is　the　cause　through　which　A；　the　final　cause；　inheres　in　C？
  It　is　B；　the　non…regurgitation　of　food；　but　B　is　a　kind　of
  definition　of　A；　for　A　will　be　explained　by　it。　Why　is　B　the　cause
  of　A's　belonging　to　C？　Because　t