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 significance.

Sec.2. The Division of Labor.  There is one such truth in the economic sphere which it is essential to appreciate vividly and fully, with the widest sweep of the imagination and the sharpest clarity of thought.  Man lives by cooperating with his fellow-men.  In the modern world, that cooperation is of a boundless range and an indescribable complexity.  Yet it is essentially undesigned and uncontrolled by man.  The humblest inhabitant of the United States or Great Britain depends for the satisfaction of his simplest needs upon the activities of innumerable people, in every walk of life and in every corner of the globe.  The ordinary commodities which appear upon his dinner table represent the final product