DEMAND

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things  as  units  of  utility  the  same  for  me
as  for  any  other  person,  the  marginal  utility
of  a  thing  for  which  he  and  I  paid  the  same
price  would  not  necessarily  be  the  same  for
both  of  us  since  we  might  differ  in  respect  of
tastes  and  intensity,  of  wants  and  he  might
be  the  wealthier.  As  a  person  gets  wealthier,
the  marginal  utility  of  money  to  him  falls,
other  things  being  equal.  The  conclusion
to  which  the  argument  of  this  paragraph
meanders  is  that  the  surplus  in  utility  enjoyed
by  a  group  of  consumers  of  a  commodity  is
an  assemblage  of  diverse  things  which  cannot
be  added  to  one  another  to  make  up  a  thinkable ­
  whole.
Consumer’s  surplus  is  more  easily  comprehensible ­
  when  we  think  of  it  in  terms  of  money.
We  may  then  define  it  as  the  difference  between
what  a  person  does  give  for  a  thing  and  what
he  would  give  rather  than  be  deprived  of  the
thing  altogether.  If  I  would  give  £45  for  a
piano  upon  which  I  have  set  my  heart,  and
am  able  to  buy  it  for  £20,  I  enjoy  a  consumer’s
surplus  of  £25  on  my  acquisition.  The  £25
stands  for  what  I  have  saved  in  view  of  the
fact  that  I  was  not  compelled  to  give  as  much
for  the  piano  as  I  was  prepared  to  give.  Similarly ­
  we  may  interpret  the  surplus  in  the  case
of  the  apples  already  treated  in  terms  of  utility.