48

POLITICAL  ECONOMY

of  the  subtlest  generalisations  wrung  by
Dr.  Marshall  out  of  the  facts  of  experience
after  analysing  them  to  the  last  dregs  by  the
marginal  method.  The  conception  is  known
as  that  of  consumers’  surplus.
Consumers’  surplus  has  been  used  in  two
ways  :  to  indicate  on  the  one  hand  a  surplus
of  utility  and  on  the  other  hand  the  expression
of  this  in  terms  of  money.  The  “  surplus  ”
in  utility  obtained  by  an  individual  from
anything,  consists  in  the  utility  which  he
obtains  from  that  thing  over  and  above  the
product  of  its  marginal  utility  and  the  quantity
of  it  that  he  consumes.  It  is  called  a  surplus
because  it  represents  a  gain  which,  so  to
speak,  the  individual  gets  for  nothing.  Let
us  take  a  particular  example.  Suppose  a
person  consumes  a  dozen  apples  a  week.
The  price  he  pays  for  apples  measures  the
difference  between  the  utility  to  him  of
twelve  apples  and  the  utility  to  him  of  eleven
apples,  in  short,  the  marginal  utility  of  the
apples.  He  buys  the  twelfth  apple  because
he  thinks  it  is  just  worth  his  while  to  do
so.  For  the  sake  of  clearness  of  statement
let  us  say  that  the  marginal  utility  of  apples
contains  ten  units  of  utility—carefully  bearing ­
  in  mind  at  the  same  time  that  there  are
no  such  things  as  units  of  utility.  Then,  if