controlling　them：　but　that　they　may　be　legitimately　controlled　for
  these　ends　is　in　principle　undeniable。　On　the　other　hand；　there　are
  questions　relating　to　interference　with　trade　which　are　essentially
  questions　of　liberty；　such　as　the　Maine　Law；　already　touched　upon；　the
  prohibition　of　the　importation　of　opium　into　China；　the　restriction　of
  the　sale　of　poisons；　all　cases；　in　short；　where　the　object　of　the
  interference　is　to　make　it　impossible　or　difficult　to　obtain　a
  particular　commodity。　These　interferences　are　objectionable；　not　as
  infringements　on　the　liberty　of　the　producer　or　seller；　but　on　that　of
  the　buyer。
  One　of　these　examples；　that　of　the　sale　of　poisons；　opens　a　new
  question；　the　proper　limits　of　what　may　be　called　the　functions　of
  police；　how　far　liberty　may　legitimately　be　invaded　for　the　prevention
  of　crime；　or　of　accident。　It　is　one　of　the　undisputed　functions　of
  government　to　take　precautions　against　crime　before　it　has　been
  committed；　as　well　as　to　detect　and　punish　it　afterwards。　The
  preventive　function　of　government；　however；　is　far　more　liable　to　be
  abused；　to　the　prejudice　of　liberty；　than　the　punitory　function；…　for
  there　is　hardly　any　part　of　the　legitimate　freedom　of　action　of　a
  human　being　which　would　not　admit　of　being　represented；　and　fairly
  too；　as　increasing　the　facilities　for　some　form　or　other　of
  delinquency。　Nevertheless；　if　a　public　authority；　or　even　a　private
  person；　sees　any　one　evidently　preparing　to　commit　a　crime；　they　are
  not　bound　to　look　on　inactive　until　the　crime　is　committed；　but　may
  interfere　to　prevent　it。　If　poisons　were　never　bought　or　used　for
  any　purpose　except　the　commission　of　murder　it　would　be　right　to
  prohibit　their　manufacture　and　sale。　They　may；　however；　be　wanted
  not　only　for　innocent　but　for　useful　purposes；　and　restrictions　cannot
  be　imposed　in　the　one　case　without　operating　in　the　other。　Again；　it
  is　a　proper　office　of　public　authority　to　guard　against　accidents。
  If　either　a　public　officer　or　any　one　else　saw　a　person　attempting
  to　cross　a　bridge　which　had　been　ascertained　to　be　unsafe；　and　there
  were　no　time　to　warn　him　of　his　danger；　they　might　seize　him　and
  turn　him　back；　without　any　real　infringement　of　his　liberty；　for
  liberty　consists　in　doing　what　one　desires；　and　he　does　not　desire
  to　fall　into　the　river。　Nevertheless；　when　there　is　not　a　certainty；
  but　only　a　danger　of　mischief；　no　one　but　the　person　himself　can　judge
  of　the　sufficiency　of　the　motive　which　may　prompt　him　to　incur　the
  risk：　in　this　case；　therefore　（unless　he　is　a　child；　or　delirious；
  or　in　some　state　of　excitement　or　absorption　incompatible　with　the
  full　use　of　the　reflecting　faculty）；　he　ought；　I　conceive；　to　be
  only　warned　of　the　danger；　not　forcibly　prevented　from　exposing
  himself　to　it。　Similar　considerations；　applied　to　such　a　question　as
  the　sale　of　poisons；　may　enable　us　to　decide　which　among　the
  possible　modes　of　regulation　are　or　are　not　contrary　to　principle。
  Such　a　precaution；　for　example；　as　that　of　labelling　the　drug　with
  some　word　expressive　of　its　dangerous　character；　may　be　enforced
  without　violation　of　liberty：　the　buyer　cannot　wish　not　to　know　that
  the　thing　he　possesses　has　poisonous　qualities。　But　to　require　in
  all　cases　the　certificate　of　a　medical　practitioner　would　make　it
  sometimes　impossible；　always　expensive；　to　obtain　the　article　for
  legitimate　uses。
  The　only　mode　apparent　to　me；　in　which　difficulties　may　be　thrown　in
  the　way　of　crime　committed　through　this　means；　without　any
  infringement　worth　taking　into　account　upon　the　liberty　of　those　who
  desire　the　poisonous　substance　for　other　purposes；　consists　in
  providing　what；　in　the　apt　language　of　Bentham；　is　called
  〃preappointed　evidence。〃　This　provision　is　familiar　to　every　one　in
  the　case　of　contracts。　It　is　usual　and　right　that　the　law；　when　a
  contract　is　entered　into；　should　require　as　the　condition　of　its
  enforcing　performance；　that　certain　formalities　should　be　observed；
  such　as　signatures；　attestation　of　witnesses；　and　the　like；　in　order
  that　in　case　of　subsequent　dispute　there　may　be　evidence　to　prove　that
  the　contract　was　really　entered　into；　and　that　there　was　nothing　in
  the　circumstances　to　render　it　legally　invalid：　the　effect　being　to
  throw　great　obstacles　in　the　way　of　fictitious　contracts；　or　contracts
  made　in　circumstances　which；　if　known；　would　destroy　their　validity。
  Precautions　of　a　similar　nature　might　be　enforced　in　the　sale　of
  articles　adapted　to　be　instruments　of　crime。　The　seller；　for
  example；　might　be　required　to　enter　in　a　register　the　exact　time　of
  the　transaction；　the　name　and　address　of　the　buyer；　the　precise
  quality　and　quantity　sold；　to　ask　the　purpose　for　which　it　was　wanted；
  and　record　the　answer　he　received。　When　there　was　no　medical
  prescription；　the　presence　of　some　third　person　might　be　required；
  to　bring　home　the　fact　to　the　purchaser；　in　case　there　should
  afterwards　be　reason　to　believe　that　the　article　had　been　applied　to
  criminal　purposes。　Such　regulations　would　in　general　be　no　material
  impediment　to　obtaining　the　article；　but　a　very　considerable　one　to
  making　an　improper　use　of　it　without　detection。
  The　right　inherent　in　society；　to　ward　off　crimes　against　itself
  by　antecedent　precautions；　suggests　the　obvious　limitations　to　the
  maxim；　that　purely　self…regarding　misconduct　cannot　properly　be
  meddled　with　in　the　way　of　prevention　or　punishment。　Drunkenness；
  for　example；　in　ordinary　cases；　is　not　a　fit　subject　for　legislative
  interference；　but　I　should　deem　it　perfectly　legitimate　that　a　person；
  who　had　once　been　convicted　of　any　act　of　violence　to　others　under　the
  influence　of　drink；　should　be　placed　under　a　special　legal
  restriction；　personal　to　himself；　that　if　he　were　afterwards　found
  drunk；　he　should　be　liable　to　a　penalty；　and　that　if　when　in　that
  state　he　committed　another　offence；　the　punishment　to　which　he　would
  be　liable　for　that　other　offence　should　be　increased　in　severity。
  The　making　himself　drunk；　in　a　person　whom　drunkenness　excites　to　do
  harm　to　others；　is　a　crime　against　others。　So；　again；　idleness；　except
  in　a　person　receiving　support　from　the　public；　or　except　when　it
  constitutes　a　breach　of　contract；　cannot　without　tyranny　be　made　a
  subject　of　legal　punishment；　but　if；　either　from　idleness　or　from
  any　other　avoidable　cause；　a　man　fails　to　perform　his　legal　duties
  to　others；　as　for　instance　to　support　his　children；　it　is　no　tyranny
  to　force　him　to　fulfil　that　obligation；　by　compulsory　labour；　if　no
  other　means　are　available。
  Again；　there　are　many　acts　which；　being　directly　injurious　only　to
  the　agents　themselves；　ought　not　to　be　legally　interdicted；　but　which；
  if　done　publicly；　are　a　violation　of　good　manners；　and　coming　thus
  within　the　category　of　offences　against　others；　may　rightly　be
  prohibited。　Of　this　kind　are　offences　against　decency；　on　which　it
  is　unnecessary　to　dwell；　the　rather　as　they　are　only　connected
  indirectly　with　our　subject；　the　objection　to　publicity　being
  equally　strong　in　the　case　of　many　actions　not　in　themselves
  condemnable；　nor　supposed　to　be　so。
  There　is　another　question　to　which　an　answer　must　be　found；
  consistent　with　the　principles　which　have　been　laid　down。　In　cases
  of　personal　conduct　supposed　to　be　blamable；　but　which　respect　for
  liberty　precludes　society　from　preventing　or　punishing；　because　the
  evil　directly　resulting　falls　wholly　on　the　agent；　what　the　agent　is
  free　to　do；　ought　other　persons　to　be　equally　free　to　counsel　or
  instigate？　This　question　is　not　free　from　difficulty。　The　case　of　a
  person　who　solicits　another　to　do　an　act　is　not　strictly　a　case　of
  self…regarding　conduct。　To　give　advice　or　offer　inducements　to　any　one
  is　a　social　act；　and　may；　therefore；　like　actions　in　general　which
  affect　others；　be　supposed　amenable　to　social　control。　But　a　little
  reflection　corrects　the　first　impression；　by　showing　that　if　the
  case　is　not　strictly　within　the　definition　of　individual　liberty；
  yet　the　reasons　on　which　the　principle　of　individual　liberty　is
  grounded　are　applicable　to　it。　If　people　must　be　allowed；　in
  whatever　concerns　only　themselves；　to　act　as　seems　best　to　themselves；
  at　their　own　peril；　they　must　equally　be　free　to　consult　with　one
  another　about　what　is　fit　to　be　so　done；　to　exchange　opinions；　and
  give　and　receive　suggestions。　Whatever　it　is　permitted　to　do；　it
  must　be　permitted　to　advise　to　do。　The　question　is　doubtful　only
  when　the　instigator　derives　a　personal　benefit　from　his　advice；　when
  he　makes　it　his　occupation；　for　subsistence　or　pecuniary　gain；　to
  promote　what　society　and　the　State　consider　to　be　an　evil。　Then；
  indeed；　a　new　element　of　complication　is