At first this question of the so-called personal equation remained a purely practical astronomical problem. But a few decades later it gave rise to two classes of investigations of psychological importance, both of the experimental kind. The first was an investigation of a comparatively simple problem--the duration of the mental processes. Among such processes measured were the simple perception, the discrimination of several perceptions, the simple reaction to them, the reproduction of any suggested idea, the reproduction of a specific suggested idea, and so forth. Not only was the duration of these processes studied, but also their dependence on differences of stimulation, the accompanying circumstances, the individual differences, the subject’s trend of thought. The second class of investigations was concerned with the more complex mental processes of attending and willing. As examples may be mentioned inquiries into the attention of a person confronted by a multitude of impressions, a study of the order in which the several impressions are perceived, a determination of the largest number of impressions perceptible as a mental unit, and research into the causal relations between ideas and actions. A more recent contribution of natural science to the advancement of psychology has come from investigations in the physiology and pathology of the central nervous system since the discovery about 1870 of the so-called speech center by Broca, and of the motor areas of the brain cortex by Fritsch and Hitzig. Some have placed a rather low value on this contribution and, noticing the errors and immature conceptions of this or that investigator, have arrived at the conclusion that psychology can learn nothing worth mentioning from the work of these men. This, it seems to me, is a great mistake. Quite aside from innumerable details, psychology owes to the investigations made in recent years concerning the physiology of the brain two fundamental conceptions. In the first place it has come to be generally recognized that the search of centuries for the exact seat of the soul in the brain--for the point where mind and body come into interaction--is without an object. There is no seat of the soul in this sense; the brain is the embodiment of almost absolute decentralization. Our mind receives the impressions of the external world by means of widely separated parts of the brain, as different sensations, according to the peripheral organs stimulated. And our mind controls our actions by means of widely separated parts of the brain according to the local differences of the muscle groups which are called into action. All the parts of the brain are connected, but they function in relative independence, without being controlled from a single point. Now, it is clear that insight into this fact is of no little significance for our conception of the nature of mind.