PREFACE 
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must inquire whether the facts conform to the elaborated theorems; 
must make sure that nothing has been forgotten in the premises, nothing 
has been erroneous in the reasoning. It is incumbent on the economist 
to follow a procedure similar to that used in the natural sciences. The 
physicist or biologist who believes that he has hit on a generalization 
which conforms to the regularities of the external world uses it merely 
as a working hypothesis. He proceeds to test it by observation and 
experiment. The economist should do the same for his hypotheses. 
In economics this task is more difficult than in most natural sciences, 
because the economist is debarred from the method which has proved 
in them by far the most serviceable, that of experiment. He cannot 
experiment ; he can resort to observation only. Observation, however, 
he must utilize to the utmost — thru history, description, statistics. In 
so doing he may or may not find confirmation of his hypotheses. Quite 
probably he will find partial confirmation only; he will have occasion, 
to a greater or less extent, for revision, amendment, restatement. 
The task of verification and correction has constituted the most 
laborious and difficult portion of this book. Yet I am well aware that 
much more remains to be done, and can only hope that others will 
carry the same sort of inquiry to better fruition. The difficulties 
which I have encountered have been due in no small part to the inade- 
quacy of the available data, and especially to the lack of statistical 
material put together in such way as to throw light on the validity 
of the theoretic conclusions. Something will be gained if I have 
succeeded in calling the attention of statisticians to the problems that 
arise and to the sort of information that is needed. In every direc- 
tion — not only from the statisticians, but from the historians, annal- 
ists, men of affairs — economic science needs more and more of well 
sifted material, for the confirmation (or confutation, as the case may 
be) of its hypothetical generalizations. 
In Part III, which deals with international trade under dislocated 
exchanges, I have unfortunately been compelled to confine myself 
in the main to theoretic reasoning. Even more than for Part II, the 
statistical material, abundant tho it is, does not dovetail with the 
theoretic frame work. Little of it is in such form that the hypotheses 
can be either substantiated or overthrown. I am aware, however, 
that not all that is available has been here examined; further patient 
research might have made possible substantive results of greater 
scope. A wish to present for the consideration of the expert certain