Every experience leaves the brain altered. The nerve fibers and the cells have gone into new stages of disposition for certain excitements. This disposition may be slowly lost. In that case the earlier experience cannot be reproduced; we have forgotten it. But as long as the disposition lasts--it is quite indifferent whether we conceive it more in terms of chemical changes or physical variations, as processes in the nerve cells or between the nerve cells--the physiological change alone is responsible for the awakening of the memory idea under favoring associative conditions. Of course, someone might reply: can we not fancy that there remains on the psychical side also a disposition? Each idea which we have experienced may have left a psychical trace which alone may make it possible that the idea may come back to us again. But what is really meant and what is gained by such a hypothesis? First, do not let us forget that such a proposition could only have one possible end in view, namely, the explanation of the reappearance of memories. But when we discussed the basis of physiological psychology, we convinced ourselves that mental facts as such are not causally connected anyhow. Our real inner life has its internal connections, connections of will and purpose, but as soon as we have taken that great psychological step and look on inner life as merely psychological objects, then the material is connected only through the underlying physiological processes and we can never explain causally the appearance of an idea through the preceding existence of another idea. We may expect one after the other, but we have no insight into the mechanism which makes the second follow after the first. Such insight into necessary connection we find only on the physical side, and we saw that just here lies the starting point for the modern view of physiological psychology. If that holds true for the connections between idea and idea, of course it holds true in the same way for the connection between mental disposition and the corresponding memory. We can understand causally that a chemical disposition in the nerve fibers brings about a chemical excitement in those neurons, but how a mental disposition is to create mental experience we could not understand; and to explain it casually, we should need again a reference to the underlying physiological processes. The hypothesis of mental dispositions would thus be an entirely superfluous addition by which we transcend the real experience without gaining anything for the explanation.