The  action  of  indebttcUus  assumpsit  for  the  recovery  of  money  had  and  re-
ceived, and  for  money  paid,  etc.,  is  an  action  of  the  common  law,  but  to  a
great  extent  an  equitable  action,  adopted  for  the  enforcement  of  many
equitable  as  well  as  legal  rights.  And  it  is  a  fundamental  principle  of
this  action,  that  it  lies  for  the  recovery  of  money,  which,  ex  cequo  et
boiiOy  ought  to  be  paid  over  to  the  plaintiff;  and  that  the  law,  in  case  of
such  equity,  will  imply  a  promise  to  pay  it.^  The  principles  of  the  action
were  very  definitely  stated  by  Lord  Mansfield,  in  the  leading  case  of
Moses  9.  Macferlan,*  and  have  never  since  been  doubted.  He  says,  "  If  the
defendant  be  under  an  obligation,  from  the  ties  of  natural  justice,  to  refund,
the  law  implies  a  debt,  and  gives  this  action  founded  in  the  equity  of  the
plaintiff's  case,  as  if  it  were  upon  a  contract."  He  particularizes,  and  says
again :  '*  This  kind  of  equitable  action  to  recover  back  money  which  ought
not  in  justice  to  be  kept,  is  very  beneficial,  and  therefore  much  encour-
aged." He  goes  on  to  enumerate  several  cases  in  which  money  paid  can-
not be  recovered  back,  as,  if  advanced  in  payment  of  a  debt  barred  by  the
statute  of  limitations,  etc.,  and  as  a  reason :  "  Because  in  all  these  cases
the  defendant  may  retain  it  with  a  safe  conscience,  though  by  positive  law
he  was  barred  from  recovering."  And  he  refers  to  money  paid  by  mistake,
as  an  instance  of  the  equity  which  will  sustain  the  action  ;  making  no  allu-
sion to  a  distinction  betweeu  a  mistake  of  law  and  a  mistake  of  fact,  —
a  suggestion,  we  believe,  as  applied  to  this  action,  of  a  much  more  recent
date.
