rned with, the ordinary business world; and it is, therefore,
mainly upon such facts that the theory must be based. People
sometimes speak as though they supposed the economist to start from a
few psychological assumptions (e. g. that a man is actuated mainly by
his own self-interest) and to build up his theories upon such
foundations by a process of pure reasoning. When, therefore, some
advance in the study of psychology throws into apparent disrepute such
ancient maxims about human nature, these people are disposed to
conclude that the old economic theory is exploded, since its
psychological premises have been shown to be untrue. Such an attitude
involves a complete misunderstanding not merely of economics, but of
the processes of human thought. It is quite true that the various
branches of knowledge are interrelated very intimately, and that an
advance in one will often suggest a development in another. By all
means let the economist and psychologist avoid a pedantic specialism
and let each stray into the other's province whenever he thinks
fit. But the fact remains that they are primarily concerned with
different things: and that each is most to be trusted when he is upon
his own ground. When, therefore, the economist indulges in a
generalization about psychology, even when he gives it as a reason for
an economic proposition, in nine cases out of ten the economics will
not depend upon the psychology; the psychology will rather be an
inference (and very possibly a crude and hasty one) from the economic
facts of which he is tolerably sure.

But the purpose of economic theory is not merely to describe the facts
of the economic world; it is to describe them in their proper sequence
and true perspective. It must begin with those facts which are most
general and which have the widest possible significance. Those are not
likely to be the facts which our practical experience forces most
insistently upon our notice. For it is the particular and not the
general, the differences between things rather than their
resemblances, that concern us most in daily life. Nor are we likely to
find the universal facts which we require in the sphere of public
controversy. We must rather look for them in the dark recesses of our
consciousness, where are stored those truths which are so obvious that
we hardly notice them, which are so indisputable that we seldom
examine them, which seem so trite that we are apt to miss their full
significanc