xcessive when it was transferred to the
axioms from which they professed to start. A doctrine may be true
enough to expose an error, and yet not capable of yielding definite
and precise conclusions. If I know that nothing can come out of
nothing, I am on the way to a great scientific principle and able to
confute some palpable fallacies; but I am still a very long way from
understanding the principle of the 'conservation of energy.' The truth
that scarcity meant dearness was apparently well known to Joseph in
Egypt, and applied very skilfully for his purpose. Economists have
framed a 'theory of value' which explains more precisely the way in
which this is brought about. A clear statement may be valuable to
psychologists; but for most purposes of political economy Joseph's
knowledge is quite sufficient. It is the doctrine which is really used
in practice whatever may be its ultimate justification.

The postulates, however, were taken by the economists to represent
something more than approximate statements of the fact. They imply
certain propositions which might be regarded as axioms. Men desire
wealth and prefer their own interests. The whole theory might then be
regarded as a direct deduction from the axioms. It thus seemed to
have a kind of mathematical certainty. When facts failed to conform to
the theory the difficulty could be met by speaking, as Malthus spoke,
of 'tendencies,' or by appealing to the analogy of 'friction' in
mechanics. The excuse might be perfectly valid in some cases, but it
often sanctioned a serious error. It was assumed that the formula was
still absolutely true of something, and that the check or friction was
a really separable and accidental interference. Thus it became easy to
discard, as irrelevant, objections which really applied to the
principle itself, and to exaggerate the conformity between fact and
theory. The economic categories are supposed to state the essential
facts, and the qualifications necessary to make them accurate were apt
to slip out of sight. Ricardo,[338] to mention a familiar instance,
carefully points out that the 'economic rent,' which clearly
represents an important economic category, is not to be confounded, as
in 'popular' use, with the payments actually made, which often include
much that is really profit. The distinction, however, was constantly
forgotten, and the abstract formula summarily applied to the concrete
fact.

The economists had constructed a kind o