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POLITICAL  ECONOMY

The  utilities  of  certain  things  are  closely
dependent  upon  the  supplies  that  we  possess
of  other  things,  and  as  the  supplies  of  these
other  things  become  more  abundant  the
value  of  the  first  things  might  appreciate  or
depreciate.  Consequently  we  must  not  affirm
that  to  any  individual  so  much  of  a  given
article  means  so  much  utility  quite  regardless
of  the  degree  and  nature  of  his  opulence.
Finally  it  must  be  remarked  that  the
generalisation  known  as  the  law  of  diminishing ­
  utility  is  commonly  understood  to  apply
to  the  second  case  considered  above  and  to
allege  that,  income  being  constant,  additional
supplies  of  one  article  add  to  satisfaction  at
a  diminishing  rate.
It  is  very  necessary  that  the  distinction
implied  above  between  total  utility  and
marginal  utility  should  be  made  explicit.
With  reference  to  income,  total  utility  is  the
sum  of  satisfaction  which  we  derive  from  our
aggregate  outlay,  while  marginal  utility  is
the  addition  made  to  total  utility  by  the
addition  of  another  unit  of  income,  say,  by
supplementing  income  to  the  extent  of  a
shilling  or  a  sovereign.  Any  unit  of  income
that  we  please  may  be  taken,  provided  that
we  make  it  very  small  in  relation  to  the  sum
total  of  income.  With  reference  to  particular