This is an affection which has given rise to an immense amount of discussion among physicians, philosophers, and moralists, from the earliest ages down to the present. Mental derangement has been universally considered one of the most terrible calamities which could befall an individual. The exact nature of the disease, however, was never thoroughly understood until the darkness which surrounded it was dispelled by the modem investigation of the subject. The old idea of insanity held it to be a disease of the mind or soul. This theory is no longer tenable, however, in the light of modem investigations respecting the nature of the mind and its relation to the brain. As has been elsewhere shown, mind is simply the result of the activity of the brain, although it cannot be called a secretion, as it has been termed by some. It is just as much a result of the activity of the cells of the brain, or of certain parts of it, as the bile is a result of the activity of the cells of the liver, or gastric juice of the cells of the peptic glands. So-called mental disease is really disease of the mind-producing organ, or the brain. Thus, properly speaking, insanity is not a disease of the mind, but of the brain itself. This theory is amply sustained by hundreds of post-mortem examinations which have been made at institutions for the insane, where the most thorough and full investigations of this subject have been carried forward. The general principle can now be well sustained that every case of serious mental disease is accompanied by certain definite changes in the substance and cell structure of the brain, and the amount and character of the mental disorder is exactly proportionate to the nature and location of the tissue-changes in the brain.

Insanity has been variously defined by different authors, and the great diversity in the definitions given suggests very strongly the fact that an absolutely perfect definition, which shall include all cases which properly belong under this head, without including any others, is impossible. A late writer on the subject defines insanity as being "a manifestation of disease of the brain, characterized by a general or partial derangement of one or more faculties of the mind, and in which, while consciousness is not abolished, mental freedom is perverted, weakened, or destroyed." One of the greatest obstacles which is presented in the study of insanity is the difficulty of distinguishing between natural eccentricity and real mental derangement. There is no sharp dividing line between the cases in which mental derangement may be so slight that the individual is simply said in popular phrase to have "a kink in the head" or, as in Scotland, "a bee in the bonnet," and those in which the mental disorder is so pronounced as to render the individual incompetent to perform the ordinary duties of life. In other words, it is often very difficult to say whether an individual is really insane, or whether he is exceedingly odd, or eccentric. Some have even gone so far as to say that entire sanity is much more rare than some degree of insanity. Perhaps this is an extreme view of the matter, but it may safely be said that there are far more insane people engaged in the active duties of life, following their accustomed vocations, with greater or less success, than are found within the walls of lunatic asylums.

Certain symptoms which are present in cases of insanity should be defined, in order to render a description of the disease intelligent. The principal are illusion, hallucination, delusion, incoherence and delirium.