It is the chief service of Herbart to have recognized a weak point here, and to have attempted to remedy it. “The regularity of the mental life,” he is convinced, “is fully equal to that of the movements of the stars.” Physical analogies guide him in his attempt at explanation. He regards ideas as mutually repellent structures, or, as it were, elastic bodies, assigned to a space of limited capacity, forced together and made smaller by mutual pressure, but never annihilating each other. If several ideas are simultaneously called forth, they become conflicting forces, on account of the unity of the mind, in which they are compelled to be together, and on account of the opposition which exists among them. In this struggle their clearness suffers and their influence on consciousness is impaired. However, they do not perish, but become, to the extent that they suffer, latent forces. As soon as the opposing factors lose their strength these latent forces emerge again into full consciousness out of the obscurity in which they have been buried. After making some further simple assumptions as to the strength of these interferences, Herbart concludes that two ideas are sufficient to crowd a third completely out of consciousness. To his great satisfaction he thus gains from the consideration of a simple mechanism “a solution of the most general of all psychological problems.” By this problem he means the fact that of all the knowing, thinking, wishing, which at any moment might be brought about by the proper causes, only a very small part plays a significant rôle, while the rest is not really lost. That is, he means the fact of attention. But this principle of the mutual interference of ideas is not the only one he uses. The second principle upon which his theory is based is that of association. With these two weapons he takes up the fight against the faculty psychology, and carries it to a successful end. He believes that all those activities traditionally placed side by side, even feeling and desire, can be made comprehensible as results of the mechanics of ideas.