assumed　as　primary；　and　this　will　be　ensured　if　the　term　selected　is
  predicable　of　all　the　others　but　not　all　they　of　it；　since　there
  must　be　one　such　term。　Having　assumed　this　we　at　once　proceed　in　the
  same　way　with　the　lower　terms；　for　our　second　term　will　be　the　first
  of　the　remainder；　our　third　the　first　of　those　which　follow　the　second
  in　a　'contiguous'　series；　since　when　the　higher　term　is　excluded；　that
  term　of　the　remainder　which　is　'contiguous'　to　it　will　be　primary；　and
  so　on。　Our　procedure　makes　it　clear　that　no　elements　in　the
  definable　form　have　been　omitted：　we　have　taken　the　differentia　that
  comes　first　in　the　order　of　division；　pointing　out　that　animal；　e。g。
  is　divisible　exhaustively　into　A　and　B；　and　that　the　subject　accepts
  one　of　the　two　as　its　predicate。　Next　we　have　taken　the　differentia　of
  the　whole　thus　reached；　and　shown　that　the　whole　we　finally　reach　is
  not　further　divisible…i。e。　that　as　soon　as　we　have　taken　the　last
  differentia　to　form　the　concrete　totality；　this　totality　admits　of
  no　division　into　species。　For　it　is　clear　that　there　is　no　superfluous
  addition；　since　all　these　terms　we　have　selected　are　elements　in　the
  definable　form；　and　nothing　lacking；　since　any　omission　would　have
  to　be　a　genus　or　a　differentia。　Now　the　primary　term　is　a　genus；　and
  this　term　taken　in　conjunction　with　its　differentiae　is　a　genus：
  moreover　the　differentiae　are　all　included；　because　there　is　now　no
  further　differentia；　if　there　were；　the　final　concrete　would　admit
  of　division　into　species；　which；　we　said；　is　not　the　case。
  To　resume　our　account　of　the　right　method　of　investigation：　We
  must　start　by　observing　a　set　of　similar…i。e。　specifically
  identical…individuals；　and　consider　what　element　they　have　in
  common。　We　must　then　apply　the　same　process　to　another　set　of
  individuals　which　belong　to　one　species　and　are　generically　but　not
  specifically　identical　with　the　former　set。　When　we　have　established
  what　the　common　element　is　in　all　members　of　this　second　species；
  and　likewise　in　members　of　further　species；　we　should　again　consider
  whether　the　results　established　possess　any　identity；　and　persevere
  until　we　reach　a　single　formula；　since　this　will　be　the　definition
  of　the　thing。　But　if　we　reach　not　one　formula　but　two　or　more；
  evidently　the　definiendum　cannot　be　one　thing　but　must　be　more　than
  one。　I　may　illustrate　my　meaning　as　follows。　If　we　were　inquiring　what
  the　essential　nature　of　pride　is；　we　should　examine　instances　of　proud
  men　we　know　of　to　see　what；　as　such；　they　have　in　common；　e。g。　if
  Alcibiades　was　proud；　or　Achilles　and　Ajax　were　proud；　we　should
  find　on　inquiring　what　they　all　had　in　common；　that　it　was　intolerance
  of　insult；　it　was　this　which　drove　Alcibiades　to　war；　Achilles
  wrath；　and　Ajax　to　suicide。　We　should　next　examine　other　cases；
  Lysander；　for　example；　or　Socrates；　and　then　if　these　have　in　common
  indifference　alike　to　good　and　ill　fortune；　I　take　these　two　results
  and　inquire　what　common　element　have　equanimity　amid　the
  vicissitudes　of　life　and　impatience　of　dishonour。　If　they　have　none；
  there　will　be　two　genera　of　pride。　Besides；　every　definition　is　always
  universal　and　commensurate：　the　physician　does　not　prescribe　what　is
  healthy　for　a　single　eye；　but　for　all　eyes　or　for　a　determinate
  species　of　eye。　It　is　also　easier　by　this　method　to　define　the
  single　species　than　the　universal；　and　that　is　why　our　procedure
  should　be　from　the　several　species　to　the　universal　genera…this　for
  the　further　reason　too　that　equivocation　is　less　readily　detected　in
  genera　than　in　infimae　species。　Indeed；　perspicuity　is　essential　in
  definitions；　just　as　inferential　movement　is　the　minimum　required　in
  demonstrations；　and　we　shall　attain　perspicuity　if　we　can　collect
  separately　the　definition　of　each　species　through　the　group　of
  singulars　which　we　have　established　e。g。　the　definition　of
  similarity　not　unqualified　but　restricted　to　colours　and　to　figures；
  the　definition　of　acuteness；　but　only　of　sound…and　so　proceed　to　the
  common　universal　with　a　careful　avoidance　of　equivocation。　We　may
  add　that　if　dialectical　disputation　must　not　employ　metaphors；　clearly
  metaphors　and　metaphorical　expressions　are　precluded　in　definition：
  otherwise　dialectic　would　involve　metaphors。
  14
  In　order　to　formulate　the　connexions　we　wish　to　prove　we　have　to
  select　our　analyses　and　divisions。　The　method　of　selection　consists　in
  laying　down　the　common　genus　of　all　our　subjects　of　investigation…if
  e。g。　they　are　animals；　we　lay　down　what　the　properties　are　which
  inhere　in　every　animal。　These　established；　we　next　lay　down　the
  properties　essentially　connected　with　the　first　of　the　remaining
  classes…e。g。　if　this　first　subgenus　is　bird；　the　essential
  properties　of　every　bird…and　so　on；　always　characterizing　the
  proximate　subgenus。　This　will　clearly　at　once　enable　us　to　say　in
  virtue　of　what　character　the　subgenera…man；　e。g。　or　horse…possess
  their　properties。　Let　A　be　animal；　B　the　properties　of　every　animal；　C
  D　E　various　species　of　animal。　Then　it　is　clear　in　virtue　of　what
  character　B　inheres　in　D…namely　A…and　that　it　inheres　in　C　and　E　for
  the　same　reason：　and　throughout　the　remaining　subgenera　always　the
  same　rule　applies。
  We　are　now　taking　our　examples　from　the　traditional　class…names；　but
  we　must　not　confine　ourselves　to　considering　these。　We　must　collect
  any　other　common　character　which　we　observe；　and　then　consider　with
  what　species　it　is　connected　and　what。properties　belong　to　it。　For
  example；　as　the　common　properties　of　horned　animals　we　collect　the
  possession　of　a　third　stomach　and　only　one　row　of　teeth。　Then　since　it
  is　clear　in　virtue　of　what　character　they　possess　these
  attributes…namely　their　horned　character…the　next　question　is；　to　what
  species　does　the　possession　of　horns　attach？
  Yet　a　further　method　of　selection　is　by　analogy：　for　we　cannot
  find　a　single　identical　name　to　give　to　a　squid's　pounce；　a　fish's
  spine；　and　an　animal's　bone；　although　these　too　possess　common
  properties　as　if　there　were　a　single　osseous　nature。
  15
  Some　connexions　that　require　proof　are　identical　in　that　they
  possess　an　identical　'middle'　e。g。　a　whole　group　might　be　proved
  through　'reciprocal　replacement'…and　of　these　one　class　are
  identical　in　genus；　namely　all　those　whose　difference　consists　in
  their　concerning　different　subjects　or　in　their　mode　of　manifestation。
  This　latter　class　may　be　exemplified　by　the　questions　as　to　the　causes
  respectively　of　echo；　of　reflection；　and　of　the　rainbow：　the
  connexions　to　be　proved　which　these　questions　embody　are　identical
  generically；　because　all　three　are　forms　of　repercussion；　but
  specifically　they　are　different。
  Other　connexions　that　require　proof　only　differ　in　that　the　'middle'
  of　the　one　is　subordinate　to　the　'middle'　of　the　other。　For　example：
  Why　does　the　Nile　rise　towards　the　end　of　the　month？　Because　towards
  its　close　the　month　is　more　stormy。　Why　is　the　month　more　stormy
  towards　its　close？　Because　the　moon　is　waning。　Here　the　one　cause　is
  subordinate　to　the　other。
  16
  The　question　might　be　raised　with　regard　to　cause　and　effect　whether
  when　the　effect　is　present　the　cause　also　is　present；　whether；　for
  instance；　if　a　plant　sheds　its　leaves　or　the　moon　is　eclipsed；　there
  is　present　also　the　cause　of　the　eclipse　or　of　the　fall　of　the
  leaves…the　possession　of　broad　leaves；　let　us　say；　in　the　latter　case；
  in　the　former　the　earth's　interposition。　For；　one　might　argue；　if　this
  cause　is　not　present；　these　phenomena　will　have　some　other　cause：　if
  it　is　present；　its　effect　will　be　at　once　implied　by　it…the　eclipse　by
  the　earth's　interposition；　the　fall　of　the　leaves　by　the　possession　of
  broad　leaves；　but　if　so；　they　will　be　logically　coincident　and　each
  capable　of　proof　through　the　other。　Let　me　illustrate：　Let　A　be
  deciduous　character；　B　the　possession　of　broad　leaves；　C　vine。　Now
  if　A　inheres　in　B　（for　every　broad…leaved　plant　is　deciduous）；　and　B
  in　C　（every　vine　possessing　broad　leaves）；　then　A　inheres　in　C
  （every　vine　is　deciduous）；　and　the　middle　term　B　is　the　cause。　But
  we　can　also　demonstrate　that　the　vine　has　broad　leaves　because　it　is
  deciduous。　Thus；　let　D　be　broad…leaved；　E　deciduous；　F　vine。　Then　E
  inheres　in　F　（since　every　vine　is　deciduous）；　and　D　in　E　（for　every