1. Theory and Fact. The controversy between the "Theorist" and the "Practical Man" is common to all branches of human affairs, but it is more than usually prevalent, and perhaps more than usually acrid in the economic sphere. It is always a rather foolish controversy, and I have no intention of entering into it, but its prevalence makes it desirable to emphasize a platitude. Economic theory must be based upon actual fact: indeed, it must be essentially an attempt, like all theory, to describe the actual facts in proper sequence, and in true perspective; and if it does not do this it is an imposture. Moreover, the facts which economic theory seeks to describe are primarily economic facts, facts, that is to say, which emerge in, and are concerned 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."