rtain
psychological facts, such as are implied in all statements about human
nature. But the economist, as an economist, is content to take them for
granted without investigating the ultimate psychological laws upon
which they depend. Those laws, or rather their results, are a part of
his primary data, although he may go so far into psychological problems
as to try to state them more accurately. The selfishness or
unselfishness of the economic man has to be considered by the
psychologist or by the moralist; but the economist has only to consider
their conclusions so far as they affect the facts. So long as it is
true, for example, that scarcity causes dearness, that profits attract
capital, that demand and supply tend to equalise each other, and so
forth, his reasonings are justified; and the further questions of the
ethical and psychological implications of these facts must be treated
by a different science. The question of the play of economic forces
thus generally reduces itself to a problem which may be thus stated:
What are the conditions of industrial equilibrium? How must prices,
rates of wages, and profit be related in order that the various classes
concerned may receive such proportions of produce as are compatible
with the maintenance of the existing system of organisation? If any
specified change occurs, if production becomes easier or more
difficult, if a tax be imposed, or a regulation of any kind affects
previous conditions, what changes will be necessary to restore the
equilibrium? These are the main problems of Political Economy. To
solve, or attempt to solve them, we have to describe accurately the
existing mechanism, and to suppose that it will regulate itself on the
assumption which I have indicated as to demand and supply, the flow of
capital and labour, and so forth. To go beyond these assumptions, and
to justify them by psychological and other considerations, may be and
is a most interesting task, but it takes us beyond the sphere of
Economics proper.

I must here diverge for a little, to notice the view of the school of
economists which seems to regard scientific accuracy as attainable by a
different path. Jevons, its most distinguished leader in England, says
roundly, that political science must be a "mathematical science,"
because "it deals throughout with quantities"; and we have been since
provided with a number of formulae, corresponding to this doctrine. The
obvious general reply would be,