The combinations of these elements, however, can never be judged according to the laws that are valid for the combination of the physical processes of life. If we try to do this, we eliminate what is most characteristic and important in our mental life. This reduction of the so-called principle of parallelism is occasionally called inconsequent and unsatisfying. This objection rests upon the interference of _a priori_ metaphysical theories of the past, whose principles have long been thrown aside by science, and also upon ignorance of the real problem which psychology has to solve. This problem can surely never consist in applying, in connection with psychical processes, principles which do not belong to the psychical side of life. It must much rather consist in the attempt to gain principles out of the contents of our psychical life, just as in the reverse case physiological investigation of the change of matter and energy in the organism does not in the least, and rightly so, trouble itself with the psychical qualities of the organism. For the real unity of life will not be understood by subjecting real phenomena to laws with which they have absolutely no inner relationship. No, we must try to explain all sides of life and then the relations of these to each other. From the standpoints which have here been developed as to the relation of psychical to natural laws and as to their combination into one unity, we may now decide a question which is of mythological origin, and which was transferred by mythology to philosophy and ultimately to psychology. This question is the one as to the nature of the soul. For the primitive thinker the soul was a demoniacal being, which had its seat in the whole body, but especially in certain favoured organs, such as the heart, the kidneys, the liver, or the blood. Besides this oldest idea of a body-soul, there soon arose a second idea of a soul only externally bound to the parts of the body, and this soul left the body at death in the last breath, and also for a short time during sleep, as noticed in the images of dreams. This was called the breath-soul or the shadow-soul. For a long time, in spite of the self-contradiction, these two conceptions were joined together, although we see in the development of mythological thought that the breath-soul or psyche slowly supersedes the idea of a body-soul.