I had hoped, consequently, that this book, having played a part in stimulating intelligent quantitative work in the mental and social sciences, would suffer a natural death. It is the case, how ever, that for the great majority of students of psychology, sociology and education, the abstract mathematical treatment, characteristic of the first two books mentioned, is out of question. In fact, an elementary introduction to the theory of mental measurements, treating the simpler general problems in the logic of quantitative thinking, is needed now more than ever. The increased use of modern methods in measuring conditions, differences, changes, and relations, including correlations or resemblances, requires that even those students of the mental and social sciences who will themselves never undertake original quantitative work should be able to interpret such results as the modern methods present. So this book is reissued.