INTRODUCTORY 
81 
for two reasons. It is of value, in the first 
place, because what actually happens fre 
quently approximates to the long-period 
theoretical result though it never exactly 
corresponds with it ; and in the second 
place, because no movement can be understood 
till the forces producing it are understood, 
whether a force is ever left in unrestrained 
freedom for as long as it lasts or not. 
The economics of this book will be concerned 
almost exclusively with long-period results. 
These results are known as normal. In 
contrast with them short-period results are 
sometimes described as the sub-normal. The 
reader must be particularly careful not to 
fall into the error made by a writer who 
complained that the assumption of the long- 
period expresses a “ theory of perfectibility,” 
and implies a “ possible state of perfection 
in the material world.” He judged that 
“ it is difficult to be so confident about the 
present trend of our social and economic 
strivings as to continue to use the words 
‘ in the long run ’ with any boisterous hope 
fulness.” The reader will understand that 
the phrase “ in the long run ” expresses 
neither optimism nor pessimism, but simply 
that use is being made of the kind of hypo 
thesis on which all deductive science is based.