Social thought is both concrete and abstract. Concrete thinking rarely goes deep. It asks few questions, raises few doubts, and perceives few connections. Abstract thinking seeks causal explanations, classifies concretenesses, penetrates relationships, and proposes well-balanced procedures. The distinction, however, is largely one of degree. Concrete thinking is characteristic of every normal person, but abstract perceptions are uncommon. The ability to do abstract thinking, to get at the deeper meanings of phenomena, to penetrate the mysteries of life, is rare. Concrete thinking constitutes the major sector of the thought-life of every person, nearly all the time. Here and there, however, in human history we find individuals who have been freed or who have freed themselves from the daily struggle for a living, from the race to make money, or from the selfish enticements of life-long loafing, and have joined the world of scholars, seeking to know the truth, the truth which makes men and women free--free to develop useful personalities in a vast, changing complex of human living. When man, having leisure to think abstractly, has set himself to the task of thought research, his mind has ventured along at least five pathways. (1) Man has given considerable attention to his relation to the universe. Primitive man conceived of a personal universe, peopled with spirits. Throughout human history man has been a religious being, trying to solve the problems of a universe ruled by spirits and gods or by one supreme God. This type of thought has produced polytheisms, monotheisms, theocracies. It has formulated theological creeds and led to bitter ecclesiastical controversies. It has created fears, hopes, faiths, social ideals, and sacrificial standards. (2) Irrespective of religious needs, man has endeavored to discover proper relations to his universe. He has philosophized. He has tried to reduce to terms of thought this baffling, intangible, universal environment. He has searched for a specific ground for explaining the universe. He has sought unity in change and monism in multiplicity. He has proclaimed that change itself is Lord of the universe, or perhaps he has found solace in a creative evolution. At any rate, he has sought ultimate meanings in as unbiased an interpretation of the universe as is humanly possible. (3) From the far-flung horizons of religious and philosophic theory, man has turned his thought in an opposite direction--he has directed his thought upon itself. He has maneuvered his thought processes introspectively. He has puzzled over the structure and functions of his own mind. These series of studies have led on the one hand to treatises such as the _Critique of Pure Reason_, and on the other hand to the current expressions of behavioristic psychology or of psychoanalysis.