DEMAND

35

frequently,  for  the  best  of  motives,  choose
to  do  what  we  do  not  like.
It  happens,  however,  that  it  is  exceedingly
inconvenient  to  speak  of  demand  in  terms
of  preferences,  because  bare  preferences  are
expressible  only  in  ratios,  which  are  more
awkward  to  handle  than  simple  whole  numbers. ­
  It  is,  therefore,  customary  to  write
of  the  utilities  of  things  instead  of  the  preferences ­
  for  things,  and  to  say,  when  one  thing
is  preferred  to  another,  that  the  thing  preferred
has  greater  utility  than  the  other.  For  the
sake  of  brevity  I  shall  hereafter  write  invariably ­
  about  utilities,  but  I  shall  understand
by  them  merely  the  quantitative,  objective
expressions  of  preference  ;  and  the  student
would  act  prudently  if  he  got  into  the  habit,
on  reading  about  utilities,  of  registering
a  mental  note  to  the  effect  that  they  do  not
convey  any  particular  doctrine  with  regard
to  the  determination  of  human  action,  but  are
merely  symbols  standing  for  the  facts  of  choice.
We  may  now  proceed  to  amplify  and
qualify  the  statement  that  the  utility  of
things  diminishes  as  we  get  additional  supplies
of  them.  We  had  better  begin  by  carefully
scrutinizing  the  management  of  income  as  a
whole,  and  afterwards  concentrate  on  a
microscopic  analysis  of  the  relation  between