But　much　more　of　the　meaning　even　of　these　would　have　been　understood；
  and　what　was　understood　would　have　been　far　more　deeply　impressed　on
  the　mind；　if　the　man　had　been　accustomed　to　hear　it　argued　pro　and　con
  by　people　who　did　understand　it。　The　fatal　tendency　of　mankind　to
  leave　off　thinking　about　a　thing　when　it　is　no　longer　doubtful；　is　the
  cause　of　half　their　errors。　A　contemporary　author　has　well　spoken　of
  〃the　deep　slumber　of　a　decided　opinion。〃
  But　what！　（it　may　be　asked）　Is　the　absence　of　unanimity　an
  indispensable　condition　of　true　knowledge？　Is　it　necessary　that　some
  part　of　mankind　should　persist　in　error　to　enable　any　to　realise　the
  truth？　Does　a　belief　cease　to　be　real　and　vital　as　soon　as　it　is
  generally　received…　and　is　a　proposition　never　thoroughly　understood
  and　felt　unless　some　doubt　of　it　remains？　As　soon　as　mankind　have
  unanimously　accepted　a　truth；　does　the　truth　perish　within　them？　The
  highest　aim　and　best　result　of　improved　intelligence；　it　has
  hitherto　been　thought；　is　to　unite　mankind　more　and　more　in　the
  acknowledgment　of　all　important　truths；　and　does　the　intelligence　only
  last　as　long　as　it　has　not　achieved　its　object？　Do　the　fruits　of
  conquest　perish　by　the　very　completeness　of　the　victory？
  I　affirm　no　such　thing。　As　mankind　improve；　the　number　of
  doctrines　which　are　no　longer　disputed　or　doubted　will　be　constantly
  on　the　increase：　and　the　well…being　of　mankind　may　almost　be
  measured　by　the　number　and　gravity　of　the　truths　which　have　reached
  the　point　of　being　uncontested。　The　cessation；　on　one　question　after
  another；　of　serious　controversy；　is　one　of　the　necessary　incidents
  of　the　consolidation　of　opinion；　a　consolidation　as　salutary　in　the
  case　of　true　opinions；　as　it　is　dangerous　and　noxious　when　the
  opinions　are　erroneous。　But　though　this　gradual　narrowing　of　the
  bounds　of　diversity　of　opinion　is　necessary　in　both　senses　of　the
  term；　being　at　once　inevitable　and　indispensable；　we　are　not　therefore
  obliged　to　conclude　that　all　its　consequences　must　be　beneficial。
  The　loss　of　so　important　an　aid　to　the　intelligent　and　living
  apprehension　of　a　truth；　as　is　afforded　by　the　necessity　of　explaining
  it　to；　or　defending　it　against；　opponents；　though　not　sufficient　to
  outweigh；　is　no　trifling　drawback　from；　the　benefit　of　its　universal
  recognition。　Where　this　advantage　can　no　longer　be　had；　I　confess　I
  should　like　to　see　the　teachers　of　mankind　endeavouring　to　provide　a
  substitute　for　it；　some　contrivance　for　making　the　difficulties　of　the
  question　as　present　to　the　learner's　consciousness；　as　if　they　were
  pressed　upon　him　by　a　dissentient　champion；　eager　for　his　conversion。
  But　instead　of　seeking　contrivances　for　this　purpose；　they　have　lost
  those　they　formerly　had。　The　Socratic　dialectics；　so　magnificently
  exemplified　in　the　dialogues　of　Plato；　were　a　contrivance　of　this
  description。　They　were　essentially　a　negative　discussion　of　the
  great　question　of　philosophy　and　life；　directed　with　consummate
  skill　to　the　purpose　of　convincing　any　one　who　had　merely　adopted
  the　commonplaces　of　received　opinion　that　he　did　not　understand　the
  subject…　that　he　as　yet　attached　no　definite　meaning　to　the　doctrines
  he　professed；　in　order　that；　becoming　aware　of　his　ignorance；　he　might
  be　put　in　the　way　to　obtain　a　stable　belief；　resting　on　a　clear
  apprehension　both　of　the　meaning　of　doctrines　and　of　their　evidence。
  The　school　disputations　of　the　Middle　Ages　had　a　somewhat　similar
  object。　They　were　intended　to　make　sure　that　the　pupil　understood
  his　own　opinion；　and　（by　necessary　correlation）　the　opinion　opposed　to
  it；　and　could　enforce　the　grounds　of　the　one　and　confute　those　of
  the　other。　These　last…mentioned　contests　had　indeed　the　incurable
  defect；　that　the　premises　appealed　to　were　taken　from　authority；　not
  from　reason；　and；　as　a　discipline　to　the　mind；　they　were　in　every
  respect　inferior　to　the　powerful　dialectics　which　formed　the
  intellects　of　the　〃Socratici　viri〃；　but　the　modern　mind　owes　far
  more　to　both　than　it　is　generally　willing　to　admit；　and　the　present
  modes　of　education　contain　nothing　which　in　the　smallest　degree
  supplies　the　place　either　of　the　one　or　of　the　other。　A　person　who
  derives　all　his　instruction　from　teachers　or　books；　even　if　he
  escape　the　besetting　temptation　of　contenting　himself　with　cram；　is
  under　no　compulsion　to　hear　both　sides；　accordingly　it　is　far　from　a
  frequent　accomplishment；　even　among　thinkers；　to　know　both　sides；
  and　the　weakest　part　of　what　everybody　says　in　defence　of　his
  opinion　is　what　he　intends　as　a　reply　to　antagonists。
  It　is　the　fashion　of　the　present　time　to　disparage　negative
  logic…　that　which　points　out　weaknesses　in　theory　or　errors　in
  practice；　without　establishing　positive　truths。　Such　negative
  criticism　would　indeed　be　poor　enough　as　an　ultimate　result；　but　as
  a　means　to　attaining　any　positive　knowledge　or　conviction　worthy　the
  name；　it　cannot　be　valued　too　highly；　and　until　people　are　again
  systematically　trained　to　it；　there　will　be　few　great　thinkers；　and
  a　low　general　average　of　intellect；　in　any　but　the　mathematical　and
  physical　departments　of　speculation。　On　any　other　subject　no　one's
  opinions　deserve　the　name　of　knowledge；　except　so　far　as　he　has　either
  had　forced　upon　him　by　others；　or　gone　through　of　himself；　the　same
  mental　process　which　would　have　been　required　of　him　in　carrying　on　an
  active　controversy　with　opponents。　That；　therefore；　which　when　absent；
  it　is　so　indispensable；　but　so　difficult；　to　create；　how　worse　than
  absurd　it　is　to　forego；　when　spontaneously　offering　itself！　If　there
  are　any　persons　who　contest　a　received　opinion；　or　who　will　do　so　if
  law　or　opinion　will　let　them；　let　us　thank　them　for　it；　open　our　minds
  to　listen　to　them；　and　rejoice　that　there　is　some　one　to　do　for　us
  what　we　otherwise　ought；　if　we　have　any　regard　for　either　the
  certainty　or　the　vitality　of　our　convictions；　to　do　with　much
  greater　labour　for　ourselves。
  It　still　remains　to　speak　of　one　of　the　principal　causes　which
  make　diversity　of　opinion　advantageous；　and　will　continue　to　do　so
  until　mankind　shall　have　entered　a　stage　of　intellectual　advancement
  which　at　present　seems　at　an　incalculable　distance。　We　have　hitherto
  considered　only　two　possibilities：　that　the　received　opinion　may　be
  false；　and　some　other　opinion；　consequently；　true；　or　that；　the
  received　opinion　being　true；　a　conflict　with　the　opposite　error　is
  essential　to　a　clear　apprehension　and　deep　feeling　of　its　truth。　But
  there　is　a　commoner　case　than　either　of　these；　when　the　conflicting
  doctrines；　instead　of　being　one　true　and　the　other　false；　share　the
  truth　between　them；　and　the　nonconforming　opinion　is　needed　to
  supply　the　remainder　of　the　truth；　of　which　the　received　doctrine
  embodies　only　a　part。　Popular　opinions；　on　subjects　not　palpable　to
  sense；　are　often　true；　but　seldom　or　never　the　whole　truth。　They　are　a
  part　of　the　truth；　sometimes　a　greater；　sometimes　a　smaller　part；
  but　exaggerated；　distorted；　and　disjointed　from　the　truths　by　which
  they　ought　to　be　accompanied　and　limited。　Heretical　opinions；　on　the
  other　hand；　are　generally　some　of　these　suppressed　and　neglected
  truths；　bursting　the　bonds　which　kept　them　down；　and　either　seeking
  reconciliation　with　the　truth　contained　in　the　common　opinion；　or
  fronting　it　as　enemies；　and　setting　themselves　up；　with　similar
  exclusiveness；　as　the　whole　truth。　The　latter　case　is　hitherto　the
  most　frequent；　as；　in　the　human　mind；　one…sidedness　has　always　been
  the　rule；　and　many…sidedness　the　exception。　Hence；　even　in　revolutions
  of　opinion；　one　part　of　the　truth　usually　sets　while　another　rises。
  Even　progress；　which　ought　to　superadd；　for　the　most　part　only
  substitutes；　one　partial　and　incomplete　truth　for　another；　improvement
  consisting　chiefly　in　this；　that　the　new　fragment　of　truth　is　more
  wanted；　more　adapted　to　the　needs　of　the　time；　than　that　which　it
  displaces。　Such　being　the　partial　character　of　prevailing　opinions；
  even　when　resting　on　a　true　foundation；　every　opinion　which　embodies
  somewhat　of　the　portion　of　truth　which　the　common　opinion　omits；　ought
  to　be　considered　precious；　with　whatever　amount　of　error　and　confusion
  that　truth　may　be　blended。　No　sober　judge　of　human　affairs　will　feel
  bound　to　be　indignant　because　those　who　force　on　our　notice　truths
  which　we　should　otherwise　have　overlooked；　overlook　some　of　those
  which　we　see。　Rather；　he　will　think　that　so　long　as　popular　truth　is
  one…sided；　it　is　more　desirable　than　otherwise　that　unpopular　truth
  should　have　one…sided　assertors　too；　such　being　usually　the　most
  energetic；　and　the　most　likely　to　compel　reluctant　attention　to　the
  fragment　of　wisd