and when you thoroughly comprehend that, add
to it in succession the separate effects of each of the encumbering and
interfering agencies.'[42]

[42] _Ibid._, p. 98.

But this process of mental chromolithography, though it is sometimes a
good way of learning a science, is not a way of using it; and Bagehot
gives no indication how his complex picture of man, formed from
successive layers of abstraction, is to be actually employed in
forecasting economic results.

When Jevons published his _Theory of Political Economy_ in 1871, it was
already widely felt that a simple imaginary man, or even a composite
picture made up of a series of different simple imaginary men, although
useful in answering examination questions, was of very little use in
drafting a Factory Act or arbitrating on a sliding scale of wages.
Jevons therefore based his economic method upon the variety and not the
uniformity of individual instances. He arranged the hours of labour in
a working day, or the units of satisfaction from spending money, on
curves of increase and decrease, and employed mathematical methods to
indicate the point where one curve, whether representing an imaginary
estimate or a record of ascertained facts, would cut the others to the
best advantage.

Here was something which corresponded, however roughly, to the process
by which practical people arrive at practical and responsible results. A
railway manager who wishes to discover the highest rate of charges which
his traffic will bear is not interested if he is told that the rate when
fixed will have been due to the law that all men seek to obtain wealth
with as little effort as possible, modified in its working by men's
unwillingness to break an established business habit. He wants a method
which, instead of merely providing him with a verbal 'explanation' of
what has happened, will enable him to form a quantitative estimate of
what under given circumstances will happen. He can, however, and, I
believe, now often does, use the Jevonian method to work out definite
results in half-pennies and tons from the intersection of plotted curves
recording actual statistics of rates and traffic.

Since Jevons's time the method which he initiated has been steadily
extended; economic and statistical processes have become more nearly
assimilated, and problems of fatigue or acquired skill, of family
affection and personal thrift, of management by the _entrepreneur_ or
the paid official, have be