The scientific psychologists started on a new road yesterday. For a long time their chief interest was to study the laws of the mind. The final goal was a textbook which would contain a system of laws to which every human mind is subjected. But in recent times a change has set in. The trend of much of the best work nowadays is toward the study of individual differences. The insight into individual personalities was indeed curiously neglected in modern psychology. This does not mean that the declaration of psychological independence insisted that all men are born equal, nor did any psychologist fancy that education or social surroundings could form all men in equal moulds. But as scientists they felt no particular interest in the richness of colours and tints. They intentionally neglected the question of how men differ, because they were absorbed by the study of the underlying laws which must hold for every one. It is hardly surprising that the psychologists chose this somewhat barren way; it was a kind of reaction against the fantastic flights of the psychology of olden times. Speculations about the soul had served for centuries. Metaphysics had reigned and the observation of the real facts of life and experience had been disregarded. When the new time came in which the psychologists were fascinated by the spirit of scientific method and exact study of actual facts, the safest way was for them to imitate the well-tested and triumphant procedures of natural science. The physicist and the chemist seek the laws of the physical universe, and the psychologist tried to act like them, to study the elements from which the psychical universe is composed and to find the laws which control them. But while it was wise to make the first forward march in this one direction, the psychologist finally had to acknowledge that a no less important interest must push him on an opposite way. The human mind is not important to us only as a type. Every social aim reminds us that we must understand the individual personality. If we deal with children in the classroom or with criminals in the courtroom, with customers in the market or with patients in the hospital, we need not only to know what is true of every human being; we must above all discover how the particular individual is disposed and composed, or what is characteristic of special groups, nations, races, sexes, and ages. It is clear that new methods were needed to approach these younger problems of scientific psychology, but the scientists have eagerly turned with concerted efforts toward this unexplored region and have devoted the methods of test experiments, of statistics, and of laboratory measurements to the examination of such differences between various individuals and groups.