SUPPLY  AND  DEMAND

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ruination  of  the  results  of  such  efforts,  nature
plays  a  predominant  part,  and  the  best  and
most  conveniently  situated  natural  resources
are  strictly  limited  in  quantity.
It  is  hardly  needful  to  say  that  the  realistic
laws  of  returns  and  the  abstract  laws  of  returns
supplement  one  another.  They  both  relate
to  the  same  facts,  broadly  speaking;  but
while  the  realistic  laws  have  a  more  practical
value  in  adding  directly  to  our  information
about  economic  affairs,  as  the  latter  immediately ­
  present  themselves  to  us,  it  is  the
abstract  laws  which  have  the  greater  speculative ­
  value  in  satisfying  our  minds  as  to  the
causes  of  things.  One  defect  of  the  realistic
laws  is  that  they  cannot  be  universally
affirmed.  We  have  to  introduce  such  qualifying ­
  phrases  as  “usually”  or  “generally.”
The  abstract  laws  attain  absolute  universality,
but  only  at  the  sacrifice  of  immediate  applicability. ­

All  this  is  a  digression,  though  a  necessary
digression.  We  may  now  pick  up  the  main
thread  of  our  argument,  and  at  once  hasten  to
unite  it  with  our  earlier  exposition  of  demand.
The  supplies  of  reproducible  things  we  have
seen  depending  upon  ruling  prices.  When
constant  returns  is  found  there  is  one  supply