and mutton are produced together as their Joint Supply aspect, and the
fact that these proportions can be varied as their Composite Demand
aspect; and the question as to whether an increased demand for mutton
will increase the supply of wool turns upon whether the former aspect
is more important than the latter. Similarly labor and machinery,
employed together for the same purpose, form an instance of Joint
Demand; but in so far as they can be substituted for one another, they
constitute a Composite Supply of alternative agents of production.

These four relations of Joint Demand, Joint Supply, Composite Demand
and Composite Supply are well worth remembering and distinguishing
from one another. They are of immense importance in every branch of
economic affairs. There are hardly any economic problems upon which we
are fitted to express an opinion, unless we have a lively sense of the
far-reaching ramifications of cause and consequence, of the subtle and
often unexpected interconnections between different industries and
different markets. To gape at these complexities in a confused stupor
is as foolish as it is to ignore them. But confusion and stupor are
only too likely to represent our final state of mind, if we attempt to
deal with these complications, one by one as they occur to us, in a
piecemeal and haphazard fashion. We need a clear method, a systematic
plan by which we may search them out, and fit them into place. The
four relations which we have enumerated supply us with such a plan and
method. For they represent something more than a series of pompous
names for familiar notions. They constitute a classification of the
various ways in which the demand and supply of one thing can affect
the demand and supply of others; a classification which is exhaustive
when we add the relation of derived demand, and an analogous relation
on the supply side which we must now notice.