DEMAND

55

surplus  got  from  a  thing,  regarded  as  a
saving  of  a  part  of  what  would  have  been
paid  for  the  thing  had  the  payment  of  its  full
value  been  unavoidable,  is  spent  on  other
things  and  so  becomes  merged  in  real  income,
that  is  income  not  in  money  but  in  goods.  And
it  is  true,  admittedly,  that  we  must  not
thoughtlessly  add  up  the  surpluses  got  by  an
individual  from  different  things  and  call  the
sum  his  total  consumer’s  surplus.  Take
away  one  article  from  a  person  and  his
demand  for  certain  other  articles  will  be
increased,  while  his  demand  for  certain
other  articles  will  be  decreased.  Some
articles  are  so  closely  related  in  demand  that
their  utilities  are  commingled—and  in  some
degree  the  utilities  of  all  things  are  commingled. ­
  One  thing  helps  to  create  the  utility
of  another  thing,  as  milk  does  that  of  tea  ;  in
which  case  they  are  said  to  be  complements.
Again  one  thing  undermines  the  utility  of
another  thing,  as  tea  does  that  of  coffee—-without
  tea  we  should  prize  coffee  more  highly
—in  which  case  they  are  called  substitutes.
Nevertheless  where  it  is  possible  to  make
due  allowance  for  the  actions  and  reactions
of  things  on  one  another’s  value  and  get  at
the  aggregate  consumer’s  surplus  with  reference ­
  to  some  individual  there  would  be  a