Indeed it may be said that psychotherapy is the last word of a naturalistic age, because psychotherapy finds its real stronghold in a systematic study of the mental laws, and such study of mental laws, psychology, must indeed be the ultimate outcome of a naturalistic view of the world. Realism begins with the analysis of lifeless nature, begins with the study of the stars and the stones, of masses and of atoms. At a higher level, it turns then to the living organism, studies plants and animals and even brings the human organism entirely under the point of view of natural law. When science has thus mastered the whole physical universe, it finally brings even the mental life of man under the naturalistic point of view, treats his inner experiences like any outer objects, tears them in pieces, analyzes them, and studies them as functions of the nervous system. A scientific psychology is thus reached which is the climax of realism, because it means that even the ideas and emotions and volitions of man are treated as natural phenomena, that their causes are sought and that their effects are determined, that their laws are found out. To apply this realistic knowledge of the mind in the interest of therapy is merely to use it in the same way in which the engineer uses his knowledge of physics, when he wants to harness outer nature. As that is possible only when theoretical science has reached a certain height of development, it can indeed be said that practical psychotherapy on a scientific basis can be considered almost as the ultimate point of a realistic movement; it cannot set in until psychology has reached high development, and psychology cannot set in unless biology has preceded it. There is no doubt that we are still far from this last phase of the realistic period. The practical application of scientific psychology is still a new problem. Experimental psychology began about twenty-five years ago; at that time there existed one psychological laboratory. To-day there is no university in the world which does not have a psychological workshop. But laboratories for applied psychology are only arising in these present days, and the systematic application of scientific psychology to education and law and industry and social life and medicine is almost at its beginning. While the height of the last realistic wave was in the period of the sixties, seventies, and eighties, of the last century, its last phase, the practical application of physiological psychology, including psychotherapy, is only at its commencement.