that Political Economy cannot be an
exact science because it also deals throughout with human desires. The
objection is not simply that our data are too vague. That objection, as
Jevons says, would, perhaps, apply to meteorology, of which nobody
doubts that it is capable of being made an exact science. But why does
nobody doubt that meteorology might become an exact science? Because we
are convinced that all the data which would be needed are expressible
in precise terms of time and space; we have to do with volumes, and
masses, and weights, and forces which can be exactly measured by lines;
and, in short, with things which could be exactly measured and counted.
The data are, at present, insufficiently known, and possibly the
problems which would result might be too complex for our powers of
calculation. Still, if we could once get the data, we could express all
relevant considerations by precise figures and numbers.

Now, is this true of economic science? Within certain limits, it is
apparently true: Ricardo used mathematical formulae, though he kept to
arithmetic, instead of algebra. When Malthus spoke of arithmetical and
geometrical ratios, the statement, true or false, was, of course,
capable of precise numerical expression, so soon as the ratios were
assigned. So there was the famous formula proving a relation between
the number of quarters of corn produced by a given harvest, and the
number of shillings that would be given for a quarter of corn. If,
again, we took the number of marriages corresponding to a given price
of corn, we should obtain a formula connecting the number of marriages
with the number of quarters of corn produced. The utility of
statistics, of course, depends upon the fact that we do empirically
discover some tolerably constant and simple numerical formulae. Such
statistical statements are useful, indeed, not only in economical, but
in other inquiries, which are clearly beyond the reach of mathematics.
The proportion of criminals in a given population, the number of
suicides, or of illegitimate births, may throw some light upon judicial
and political, and even religious or ethical problems. Nor are such
formulae useless simply because empirical. The law of gravitation, for
example, is empirical. Nobody knows the cause of the observed tendency
of bodies to gravitate to each other, and therefore no one can say how
far the law which represents the tendency must be universal. Still, the
fact that, s