reasoning　is　what　logicians　call　a　syllogism；　and　has　all　its
  various　parts　and　terms；its　major　premiss；　its　minor　premiss　and
  its　conclusion。　　And；　by　the　help　of　further　reasoning；　which；　if
  drawn　out；　would　have　to　be　exhibited　in　two　or　three　other
  syllogisms；　you　arrive　at　your　final　determination；　〃I　will　not
  have　that　apple。〃　　So　that；　you　see；　you　have；　in　the　first　place；
  established　a　law　by　induction；　and　upon　that　you　have　founded　a
  deduction；　and　reasoned　out　the　special　particular　case。　　Well　now；
  suppose；　having　got　your　conclusion　of　the　law；　that　at　some　time
  afterwards；　you　are　discussing　the　qualities　of　apples　with　a
  friend：　you　will　say　to　him；　〃It　is　a　very　curious　thing；but　I
  find　that　all　hard　and　green　apples　are　sour！〃　　Your　friend　says　to
  you；　〃But　how　do　you　know　that？〃　　You　at　once　reply；　〃Oh；　because　I
  have　tried　them　over　and　over　again；　and　have　always　found　them　to
  be　so。〃　　Well；　if　we　were　talking　science　instead　of　common　sense；
  we　should　call　that　an　experimental　verification。　　And；　if　still
  opposed；　you　go　further；　and　say；　〃I　have　heard　from　the　people　in
  Somersetshire　and　Devonshire；　where　a　large　number　of　apples　are
  grown；　that　they　have　observed　the　same　thing。　　It　is　also　found　to
  be　the　case　in　Normandy；　and　in　North　America。　　In　short；　I　find　it
  to　be　the　universal　experience　of　mankind　wherever　attention　has
  been　directed　to　the　subject。〃　　Whereupon；　your　friend；　unless　he
  is　a　very　unreasonable　man；　agrees　with　you；　and　is　convinced　that
  you　are　quite　right　in　the　conclusion　you　have　drawn。　　He　believes；
  although　perhaps　he　does　not　know　he　believes　it；　that　the　more
  extensive　verifications　are；that　the　more　frequently　experiments
  have　been　made；　and　results　of　the　same　kind　arrived　at；that　the
  more　varied　the　conditions　under　which　the　same　results　are
  attained；　the　more　certain　is　the　ultimate　conclusion；　and　he
  disputes　the　question　no　further。　　He　sees　that　the　experiment　has
  been　tried　under　all　sorts　of　conditions；　as　to　time；　place；　and
  people；　with　the　same　result；　and　he　says　with　you；　therefore；　that
  the　law　you　have　laid　down　must　be　a　good　one；　and　he　must　believe
  it。
  In　science　we　do　the　same　thing；the　philosopher　exercises
  precisely　the　same　faculties；　though　in　a　much　more　delicate
  manner。　　In　scientific　inquiry　it　becomes　a　matter　of　duty　to
  expose　a　supposed　law　to　every　possible　kind　of　verification；　and
  to　take　care；　moreover；　that　this　is　done　intentionally；　and　not
  left　to　a　mere　accident；　as　in　the　case　of　the　apples。　　And　in
  science；　as　in　common　life；　our　confidence　in　a　law　is　in　exact
  proportion　to　the　absence　of　variation　in　the　result　of　our
  experimental　verifications。　　For　instance；　if　you　let　go　your　grasp
  of　an　article　you　may　have　in　your　hand；　it　will　immediately　fall
  to　the　ground。　　That　is　a　very　common　verification　of　one　of　the
  best　established　laws　of　naturethat　of　gravitation。　　The　method
  by　which　men　of　science　establish　the　existence　of　that　law　is
  exactly　the　same　as　that　by　which　we　have　established　the　trivial
  proposition　about　the　sourness　of　hard　and　green　apples。　　But　we
  believe　it　in　such　an　extensive；　thorough；　and　unhesitating　manner
  because　the　universal　experience　of　mankind　verifies　it；　and　we　can
  verify　it　ourselves　at　any　time；　and　that　is　the　strongest　possible
  foundation　on　which　any　natural　law　can　rest。
  So　much；　then；　by　way　of　proof　that　the　method　of　establishing　laws
  in　science　is　exactly　the　same　as　that　pursued　in　common　life。　　Let
  us　now　turn　to　another　matter　（though　really　it　is　but　another
  phase　of　the　same　question）；　and　that　is；　the　method　by　which；　from
  the　relations　of　certain　phenomena；　we　prove　that　some　stand　in　the
  position　of　causes　towards　the　others。
  I　want　to　put　the　case　clearly　before　you；　and　I　will　therefore
  show　you　what　I　mean　by　another　familiar　example。　　I　will　suppose
  that　one　of　you；　on　coming　down　in　the　morning　to　the　parlor　of
  your　house；　finds　that　a　tea…pot　and　some　spoons　which　had　been
  left　in　the　room　on　the　previous　evening　are　gone；the　window　is
  open；　and　you　observe　the　mark　of　a　dirty　hand　on　the　window…frame；
  and　perhaps；　in　addition　to　that；　you　notice　the　impress　of　a　hob…
  nailed　shoe　on　the　gravel　outside。　　All　these　phenomena　have　struck
  your　attention　instantly；　and　before　two　seconds　have　passed　you
  say；　〃Oh；　somebody　has　broken　open　the　window；　entered　the　room；
  and　run　off　with　the　spoons　and　the　tea…pot！〃　　That　speech　is　out
  of　your　mouth　in　a　moment。　　And　you　will　probably　add；　〃I　know
  there　has；　I　am　quite　sure　of　it！〃　　You　mean　to　say　exactly　what
  you　know；　but　in　reality　you　are　giving　expression　to　what　is；　in
  all　essential　particulars；　an　hypothesis。　　You　do　not　KNOW　it　at
  all；　it　is　nothing　but　an　hypothesis　rapidly　framed　in　your　own
  mind。　　And　it　is　an　hypothesis　founded　on　a　long　train　of
  inductions　and　deductions。
  What　are　those　inductions　and　deductions；　and　how　have　you　got　at
  this　hypothesis？　　You　have　observed　in　the　first　place；　that　the
  window　is　open；　but　by　a　train　of　reasoning　involving　many
  inductions　and　deductions；　you　have　probably　arrived　long　before　at
  the　general　lawand　a　very　good　one　it　isthat　windows　do　not
  open　of　themselves；　and　you　therefore　conclude　that　something　has
  opened　the　window。　　A　second　general　law　that　you　have　arrived　at
  in　the　same　way　is；　that　tea…pots　and　spoons　do　not　go　out　of　a
  window　spontaneously；　and　you　are　satisfied　that；　as　they　are　not
  now　where　you　left　them；　they　have　been　removed。　　In　the　third
  place；　you　look　at　the　marks　on　the　windowsill；　and　the　shoe…marks
  outside；　and　you　say　that　in　all　previous　experience　the　former
  kind　of　mark　has　never　been　produced　by　anything　else　but　the　hand
  of　a　human　being；　and　the　same　experience　shows　that　no　other
  animal　but　man　at　present　wears　shoes　with　hob…nails　in　them　such
  as　would　produce　the　marks　in　the　gravel。　　I　do　not　know；　even　if
  we　could　discover　any　of　those　〃missing　links〃　that　are　talked
  about；　that　they　would　help　us　to　any　other　conclusion！　　At　any
  rate　the　law　which　states　our　present　experience　is　strong　enough
  for　my　present　purpose。　　You　next　reach　the　conclusion　that；　as
  these　kind　＇89＇　of　marks　have　not　been　left　by　any　other　animal　than
  man；　or　are　liable　to　be　formed　in　any　other　way　than　a　man's　hand
  and　shoe；　the　marks　in　question　have　been　formed　by　a　man　in　that
  way。　　You　have；　further；　a　general　law；　founded　on　observation　and
  experience；　and　that；　too；　is；　I　am　sorry　to　say；　a　very　universal
  and　unimpeachable　one；that　some　men　are　thieves；　and　you　assume
  at　once　from　all　these　premissesand　that　is　what　constitutes　your
  hypothesisthat　the　man　who　made　the　marks　outside　and　on　the
  window…sill；　opened　the　window；　got　into　the　room；　and　stole　your
  tea…pot　and　spoons。　　You　have　now　arrived　at　a　vera　causa；you
  have　assumed　a　cause　which；　it　is　plain；　is　competent　to　produce
  all　the　phenomena　you　have　observed。　　You　can　explain　all　these
  phenomena　only　by　the　hypothesis　of　a　thief。　　But　that　is　a
  hypothetical　conclusion；　of　the　justice　of　which　you　have　no
  absolute　proof　at　all；　it　is　only　rendered　highly　probable　by　a
  series　of　inductive　and　deductive　reasonings。
  I　suppose　your　first　action；　assuming　that　you　are　a　man　of
  ordinary　common　sense；　and　that　you　have　established　this
  hypothesis　to　your　own　satisfaction；　will　very　likely　be　to　go　off
  for　the　police；　and　set　them　on　the　track　of　the　burglar；　with　the
  view　to　the　recovery　of　your　property。　　But　just　as　you　are
  starting　with　this　object；　some　person　comes　in；　and　on　learning
  what　you　are　about；　says；　〃My　good　friend；　you　are　going　on　a　great
  deal　too　fast。　　How　do　you　know　that　the　man　who　really　made　the
  marks　took　the　spoons？　　It　might　have　been　a　monkey　that　took　them；
  and　the　man　may　have　merely　looked　in　afterwards。〃　　You　would
  probably　reply；　〃Well；　that　is　all　very　well；　but　you　see　it　is
  contrary　to　all　experience　of　the　way　tea…pots　and　spoons　are
  abstracted；　so　that；　at　any　rate；　your　hypothesis　is　less　probable
  than　mine。〃　　While　you　are　talking　the　thing　over　in　this　way；
  another　friend　arrives；　one　of　the　good　kind　of　people　that　I　was
  talking　of　a　little　while　ago。　　And　he　might　say；　〃Oh；　my　dear　sir；
  you　are　certainly　going　on　a　great　deal　too　fast。　　You　are　most
  presumptuous。　　You　admit　that　all　these　occurrences　took　place　when
  you　were　fast　asleep；　at　a　time　when　you　could　not　possibly　have
  known　anything　about　what　was　taking　place。　　How　do　you　know　that
  the　laws　of　Nature　are　not　suspended　during　the　night？　　It　may　be
  that　there　has　been　some　kind　of　supernatural　interference　in　this
  case。〃　　In　point　of　fact；　he　declares　that　your　hypothesis　is　one
  of　which　you　cannot　at　all　demonstrate　the　truth；　and　that　you　are
  by　no　means　sure　that　the　laws　of　Nature　are　the　same　when　you　are
  asleep　as　when　you　are　awake。
  Well；　now；　you　cannot　at　the　moment　an