Our knowledge consists therefore of ideas, which are the results of the disturbance of the Soul-real by other Reals. These ideas live within the Soul, which is merely an indifference point where they are held together. The ideas in turn disturb and inhibit one another, and the description of our mental life is a description of the reciprocal tension of ideas. The tension among the ideas modifies the intensity of each, and consciousness of an idea is proportional to its intensity. An idea is just on the threshold of consciousness when it has the lowest degree of intensity, and is still actual. When it drops below that threshold it is changed into an impulse. The primary ideas are sensations. They are not the images of things, but the primary acts of the Soul in its attempt at self-preservation. All other mental states, like memory, imagination, feeling, and will, are to be described as kinds of tension of the ideas. Feeling and will are kinds of inhibitive tension. The coming of sensations and the interplay of sensations can be reduced to a mechanical law. Therefore, according to Herbart, psychology is the “statics and mechanics of ideas,” and must be treated mathematically. Herbart’s contribution to modern thought lies in his psychology. Modern thought has not accepted his metaphysics, but it has been influenced to a not inconsiderable degree by his psychology. Herbart gave the death-blow to the old “faculty psychology,” and he placed psychology upon the same basis as the natural sciences. The science of psychology was not to Herbart a discussion of the nature of the soul, for that is unknowable. It is the study of the aggregate of the contents of consciousness. It is not a study of psychical faculties, but of psychical elements. This reduces psychology to an atomism, like other sciences, and thereby frees it from the influence of theology. Thus was the so-called modern psychology made possible by Herbart. Herbart’s theory was also of incalculable value to modern educational theory. The conception of the influence of environment upon mental life, the theory of the development of mental life, the natural method of “preparation, presentation, association, systematization, and application” of an educational subject, the theory of the correlation of subjects――all are founded upon his psychology. Herbart’s attempt to apply mathematics to the laws of psychological phenomena was not so fortunate. At one time, during the nineteenth century, psychologists hoped much from mathematics in their science; but the hope has been practically abandoned. In recent years the demand for exactness has been met in psycho-physics, which operates with mathematics in a different way.