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POLITICAL  ECONOMY

even  this  proposition,  defining  the  principle
on  which  gold  is  distributed  between  money
and  the  arts,  is  not,  when  stated  in  general
terms,  peculiar  to  the  doctrine  of  money.
It  holds  of  all  cases  where  demand  is  composite. ­
  To  take  again  our  previous  example,
wood  would  be  so  distributed  between  housebuilding, ­
  furniture-making,  and  boat-building,
that  the  marginal  utilities  of  wood  for  all  these
purposes  would  be  the  same.
In  order  to  bring  the  theory  of  money
exactly  into  line  with  the  theory  of  the  prices
of  things,  it  must  finally  be  remarked  that
what  is  designated  purchasing-power  in  the
case  of  money  corresponds  with  what  is
designated  price  in  the  case  of  other  things.
Media  of  exchange  cannot  have  a  price  proper,
that  is  an  exchange  value  in  terms  of  money,
because  the  exchange  value  of  a  thing  cannot
be  expressed  in  terms  of  itself.  But  the
price  of  a  thing  means  the  amount  of  money
that  it  will  buy,  because  the  price  is  the  amount
of  money  which  will  buy  it,  and  exchange  is
a  simply  convertible  relation.  Now  what  a
unit  of  money  will  buy  is  called  its  purchasing
power.  Consequently,  employing  the  term
“  price  ”  broadly,  to  indicate  exchange  value
and  not  merely  exchange  value  in  money,  we
may  say  that  the  purchasing  power  of  money