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