political or physical type. About the relative importance of the
forces which produce divergencies among business cycles, however,
and about their interactions, our knowledge is meager. Perhaps we
are overlooking forces which will some day be found to play domi-
nating rôles. But the way to hasten the day of fuller understanding
is to make the best use we can of our present insights, imperfect
though they are.

Taken one at a time, most of the theories of business cycles re-
viewed in Chapter I seem plausible, not to say convincing. Certainly
each theory, this time without exception, illuminates some angle of
the problem. Taken all together, the theories render a different ser-
vice--one which is welcome only to the man who has the courage
and time to enter upon a thorough investigation. They show that
business cycles are congeries of diverse fluctuations in numerous proc-
esses--physical, psychological, and economic. Indeed, upon reflection
the theories figure less as rival explanations of a single phenomenon
than as complementary explanations of closely related phenomena.
The processes with which they severally deal are all characteristic
features of the whole. These processes not only run side by side, but
also influence and (except for the weather) are influenced by each
other. Thus the diversity of explanations, which at first seems con-
fusing, becomes an aid toward envisaging the complex character of
the problem.

Complexity is no proof of multiplicity of causes. Perhaps some
single factor is responsible for all the phenomena. An acceptable
explanation of this simple type would constitute the ideal theory of
business cycles from the practical, as well as from the scientific, view-
point. But if there be one cause of business cycles, we cannot make
sure of its adequacy as an explanation without knowing what are
the phenomena to be explained, and how the single cause produces its
complex effects, direct and indirect. Neither on the single-cause
hypothesis, nor on the hypothesis of multiple causes, are we equipped
to deal with the problem of causation until we have learned what are
the processes characteristic of business cycles, and how these processes
are related to one another. Chapter I indicated what the leading proc-

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