So true is it that certain trains of thought proceed only in limited channels of the brain, that it is found in experience, that a change of study from a subject which has long been under consideration, to another which has not been, and which is of a different character, and which requires combinations of memories and reasonings of a different order, is almost equivalent to a period of rest. After a period of application in the solution of mathematical problems, and the mind begins to weary of this study, it may, with ease and pleasure be turned to the study of some language; so that persons whose duties cover a wide range of thought and subjects of diverse character, are frequently able to employ themselves with ease more hours in the day than others whose occupations are more uniform and less diversified in character. The law of health and strength applies alike to all portions of the system, and requires a certain degree of activity in all portions, else they remain in an undeveloped or weak condition. Besides, in the case of the brain and nervous system in general, this lack of education in the direction of labor tends largely to produce too frequent self-distrust and introspection. The individual has little range of mental vision, or measure of that self-confidence which arises from ability in skilled labor, and not having any training in those activities and occupations of life which tend to develop and strengthen the nervous system; and, consequently, being unable to engage in these with any degree of satisfaction, if at all, is likely to pass into a state of self-distrust, doubt, and, after a few years, actual inability to take on any considerable measure of education of the nervous system. It is, therefore, not surprising that, in the adult period of life, when the care and responsibility of providing for others in addition to one's self, come upon such persons; and especially when periods of uncertainty and long-continued depression in the market of simple manual labor occur, individuals so illy equipped and helpless amid the competitions of society should become insane in large numbers. And I think it must be confessed that there are in operation certain tendencies, in the progress of modern civilization, which are at present very potent in their influence, as against remedies for this condition; influences which tend to push out of sight the individual, or merge him into a large whole, which moves forward for the accomplishment of purposes, regardless of the individualities which may fall out by the way.