In the same way the psychologist who aims towards explanation of mental facts must postulate that there cannot be any mental state which is not an accompaniment of a physical brain process, and is as such connected through physical means with the preceding and the following events in the psychophysical system. Only when such a general framework of theory is built up by a logical postulate, is the way open to make use of all those observations of the laboratory and of the clinic, of the zooelogist and of the anatomist. It is the theory which has to give the right setting to those scattered observations. However far we may be from being able to point to the special brain process which lies at the bottom of the higher mental state, we know beforehand that there is no shadow of an idea, no fringe of a feeling, no suggestion of a desire which does not correspond to definite processes in the brain. The details may and must be material for diverging theories, but the conflict of such hypothetical opinions has nothing to do with the certainty of the underlying conviction that if we knew the whole truth, we should recognize every single mental happening as parallel to physical processes in the nervous system. To explain mental facts means to think them as parallel to the brain processes which have their own causal connections in the physical world. We started, for instance, from the old observation that two impressions which come to our mind at the same time have a tendency to reawaken one another; and we saw that psychology was well able to formulate these facts in general statements of the association of ideas. But we realized that that in itself is not really explanation. If the odor which we smell awakes in us the name of a chemical substance, and if we now bring this under the general heading of association of ideas, an explanation is not really given by it. That smell sensation itself is not really understood as a cause of those sound sensations of the word. We have no insight into the connection of those two happenings. But the situation is entirely changed, if we consider the smell effect from the point of view of the parallelistic theory. Now the association of facts would indicate that we got the first two impressions together, because two brain processes were going on at the same time. My nose brought me the smell stimulus, my ear gave me the sound stimulus, each going on in a particular center, or, to express it in a simplified schematic way, each reaching particular brain cells, and the excitement of these brain cells being accompanied by the particular sensations. The physiologist has many possibilities of conceiving the further stages of the process, in order to satisfy the demand of explanation. He may say the excitement of each of these two brain cells, the one in the olfactory center, the other in the auditory center, irradiates in all directions through the fine branches of the brain fibers. Each cell has relations to every