22

POLITICAL  ECONOMY

religion  to  a  shoe-black,”  as  Leslie  Stephen
expressed  it—but  the  conceptions  of  demand
and  supply  were  seldom  precise,  and  were  not
infrequently  erroneous.  They  proved  as
treacherous  to  the  inexpert  as  the  conception
of  evolution  has  proved  since  to  shallow
thinkers  in  the  biological  and  mental  sciences.
Theories  tend  to  crystallise  into  formulae
expressing  half-truths,  and  these  get  to  be
fitted  unthinkingly  to  facts  to  which  they  are
alien.  In  this  connection  it  behoves  us  to
bear  in  mind  that  the  marginal  theory,  like
the  old  notions  of  demand  and  supply,  will
increasingly  cease  to  be  an  exception  to  the
rule.  Already,  indeed,  instances  could  be
recorded  of  pontifical  pronouncements,  made
on  the  strength  of  mechanical  resort  to  the
marginal  theory,  where  a  penetrating  study
of  the  living  facts  could  alone  justify  even
the  least  dogmatic  of  utterances.  To  the
limitations  of  the  use  of  the  marginal  theory
and  the  need  of  supplementing  it  invariably
with  direct  observation,  reference  will  be  made
again  and  in  more  detail.
It  must  now  be  our  aim  to  get  a  grasp
of  the  nature  and  difficulties  of  scientific
explanation  within  the  economic  domain.
It  has  been  declared  by  some  that  society