against　a　doubt　as　to　its　having　been　a　syllogistic　inference　at　all
  that　we　have　to　defend　our　argument　as　conforming　to　the　definition　of
  syllogism。　It　is　only　when　some　one　doubts　whether　the　conclusion
  proved　is　the　definable　form　that　we　have　to　defend　it　as　conforming
  to　the　definition　of　definable　form　which　we　assumed。　Hence
  syllogistic　inference　must　be　possible　even　without　the　express
  statement　of　what　syllogism　is　or　what　definable　form　is。
  The　following　type　of　hypothetical　proof　also　begs　the　question。
  If　evil　is　definable　as　the　divisible；　and　the　definition　of　a　thing's
  contrary…if　it　has　one　the　contrary　of　the　thing's　definition；　then；
  if　good　is　the　contrary　of　evil　and　the　indivisible　of　the
  divisible；　we　conclude　that　to　be　good　is　essentially　to　be
  indivisible。　The　question　is　begged　because　definable　form　is
  assumed　as　a　premiss；　and　as　a　premiss　which　is　to　prove　definable
  form。　'But　not　the　same　definable　form'；　you　may　object。　That　I　admit；
  for　in　demonstrations　also　we　premise　that　'this'　is　predicable　of
  'that'；　but　in　this　premiss　the　term　we　assert　of　the　minor　is　neither
  the　major　itself　nor　a　term　identical　in　definition；　or　convertible；
  with　the　major。
  Again；　both　proof　by　division　and　the　syllogism　just　described　are
  open　to　the　question　why　man　should　be　animal…biped…terrestrial　and
  not　merely　animal　and　terrestrial；　since　what　they　premise　does　not
  ensure　that　the　predicates　shall　constitute　a　genuine　unity　and　not
  merely　belong　to　a　single　subject　as　do　musical　and　grammatical　when
  predicated　of　the　same　man。
  7
  How　then　by　definition　shall　we　prove　substance　or　essential　nature？
  We　cannot　show　it　as　a　fresh　fact　necessarily　following　from　the
  assumption　of　premisses　admitted　to　be　facts…the　method　of
  demonstration：　we　may　not　proceed　as　by　induction　to　establish　a
  universal　on　the　evidence　of　groups　of　particulars　which　offer　no
  exception；　because　induction　proves　not　what　the　essential　nature　of　a
  thing　is　but　that　it　has　or　has　not　some　attribute。　Therefore；　since
  presumably　one　cannot　prove　essential　nature　by　an　appeal　to　sense
  perception　or　by　pointing　with　the　finger；　what　other　method　remains？
  To　put　it　another　way：　how　shall　we　by　definition　prove　essential
  nature？　He　who　knows　what　human…or　any　other…nature　is；　must　know　also
  that　man　exists；　for　no　one　knows　the　nature　of　what　does　not
  exist…one　can　know　the　meaning　of　the　phrase　or　name　'goat…stag'　but
  not　what　the　essential　nature　of　a　goat…stag　is。　But　further；　if
  definition　can　prove　what　is　the　essential　nature　of　a　thing；　can　it
  also　prove　that　it　exists？　And　how　will　it　prove　them　both　by　the　same
  process；　since　definition　exhibits　one　single　thing　and
  demonstration　another　single　thing；　and　what　human　nature　is　and　the
  fact　that　man　exists　are　not　the　same　thing？　Then　too　we　hold　that
  it　is　by　demonstration　that　the　being　of　everything　must　be
  proved…unless　indeed　to　be　were　its　essence；　and；　since　being　is　not　a
  genus；　it　is　not　the　essence　of　anything。　Hence　the　being　of
  anything　as　fact　is　matter　for　demonstration；　and　this　is　the　actual
  procedure　of　the　sciences；　for　the　geometer　assumes　the　meaning　of　the
  word　triangle；　but　that　it　is　possessed　of　some　attribute　he　proves。
  What　is　it；　then；　that　we　shall　prove　in　defining　essential　nature？
  Triangle？　In　that　case　a　man　will　know　by　definition　what　a　thing's
  nature　is　without　knowing　whether　it　exists。　But　that　is　impossible。
  Moreover　it　is　clear；　if　we　consider　the　methods　of　defining
  actually　in　use；　that　definition　does　not　prove　that　the　thing　defined
  exists：　since　even　if　there　does　actually　exist　something　which　is
  equidistant　from　a　centre；　yet　why　should　the　thing　named　in　the
  definition　exist？　Why；　in　other　words；　should　this　be　the　formula
  defining　circle？　One　might　equally　well　call　it　the　definition　of
  mountain　copper。　For　definitions　do　not　carry　a　further　guarantee　that
  the　thing　defined　can　exist　or　that　it　is　what　they　claim　to　define：
  one　can　always　ask　why。
  Since；　therefore；　to　define　is　to　prove　either　a　thing's　essential
  nature　or　the　meaning　of　its　name；　we　may　conclude　that　definition；　if
  it　in　no　sense　proves　essential　nature；　is　a　set　of　words　signifying
  precisely　what　a　name　signifies。　But　that　were　a　strange
  consequence；　for　（1）　both　what　is　not　substance　and　what　does　not
  exist　at　all　would　be　definable；　since　even　non…existents　can　be
  signified　by　a　name：　（2）　all　sets　of　words　or　sentences　would　be
  definitions；　since　any　kind　of　sentence　could　be　given　a　name；　so　that
  we　should　all　be　talking　in　definitions；　and　even　the　Iliad　would　be　a
  definition：　（3）　no　demonstration　can　prove　that　any　particular　name
  means　any　particular　thing：　neither；　therefore；　do　definitions；　in
  addition　to　revealing　the　meaning　of　a　name；　also　reveal　that　the　name
  has　this　meaning。　It　appears　then　from　these　considerations　that
  neither　definition　and　syllogism　nor　their　objects　are　identical；
  and　further　that　definition　neither　demonstrates　nor　proves
  anything；　and　that　knowledge　of　essential　nature　is　not　to　be　obtained
  either　by　definition　or　by　demonstration。
  8
  We　must　now　start　afresh　and　consider　which　of　these　conclusions　are
  sound　and　which　are　not；　and　what　is　the　nature　of　definition；　and
  whether　essential　nature　is　in　any　sense　demonstrable　and　definable　or
  in　none。
  Now　to　know　its　essential　nature　is；　as　we　said；　the　same　as　to　know
  the　cause　of　a　thing's　existence；　and　the　proof　of　this　depends　on　the
  fact　that　a　thing　must　have　a　cause。　Moreover；　this　cause　is　either
  identical　with　the　essential　nature　of　the　thing　or　distinct　from
  it；　and　if　its　cause　is　distinct　from　it；　the　essential　nature　of
  the　thing　is　either　demonstrable　or　indemonstrable。　Consequently；　if
  the　cause　is　distinct　from　the　thing's　essential　nature　and
  demonstration　is　possible；　the　cause　must　be　the　middle　term；　and；　the
  conclusion　proved　being　universal　and　affirmative；　the　proof　is　in　the
  first　figure。　So　the　method　just　examined　of　proving　it　through
  another　essential　nature　would　be　one　way　of　proving　essential　nature；
  because　a　conclusion　containing　essential　nature　must　be　inferred
  through　a　middle　which　is　an　essential　nature　just　as　a　'peculiar'
  property　must　be　inferred　through　a　middle　which　is　a　'peculiar'
  property；　so　that　of　the　two　definable　natures　of　a　single　thing
  this　method　will　prove　one　and　not　the　other。
  Now　it　was　said　before　that　this　method　could　not　amount　to
  demonstration　of　essential　nature…it　is　actually　a　dialectical　proof
  of　it…so　let　us　begin　again　and　explain　by　what　method　it　can　be
  demonstrated。　When　we　are　aware　of　a　fact　we　seek　its　reason；　and
  though　sometimes　the　fact　and　the　reason　dawn　on　us　simultaneously；
  yet　we　cannot　apprehend　the　reason　a　moment　sooner　than　the　fact；
  and　clearly　in　just　the　same　way　we　cannot　apprehend　a　thing's
  definable　form　without　apprehending　that　it　exists；　since　while　we　are
  ignorant　whether　it　exists　we　cannot　know　its　essential　nature。
  Moreover　we　are　aware　whether　a　thing　exists　or　not　sometimes
  through　apprehending　an　element　in　its　character；　and　sometimes
  accidentally；　as；　for　example；　when　we　are　aware　of　thunder　as　a　noise
  in　the　clouds；　of　eclipse　as　a　privation　of　light；　or　of　man　as　some
  species　of　animal；　or　of　the　soul　as　a　self…moving　thing。　As　often
  as　we　have　accidental　knowledge　that　the　thing　exists；　we　must　be　in　a
  wholly　negative　state　as　regards　awareness　of　its　essential　nature；
  for　we　have　not　got　genuine　knowledge　even　of　its　existence；　and　to
  search　for　a　thing's　essential　nature　when　we　are　unaware　that　it
  exists　is　to　search　for　nothing。　On　the　other　hand；　whenever　we
  apprehend　an　element　in　the　thing's　character　there　is　less
  difficulty。　Thus　it　follows　that　the　degree　of　our　knowledge　of　a
  thing's　essential　nature　is　determined　by　the　sense　in　which　we　are
  aware　that　it　exists。　Let　us　then　take　the　following　as　our　first
  instance　of　being　aware　of　an　element　in　the　essential　nature。　Let　A
  be　eclipse；　C　the　moon；　B　the　earth's　acting　as　a　screen。　Now　to　ask
  whether　the　moon　is　eclipsed　or　not　is　to　ask　whether　or　not　B　has
  occurred。　But　that　is　precisely　the　same　as　asking　whether　A　has　a
  defining　condition；　and　if　this　condition　actually　exists；　we　assert
  that　A　also　actually　exists。　Or　again