As it is my intention to present a faithful description of the various forms of mental disorder, it is necessary in the first place, to draw attention very briefly, to the classification of diseases of the mind. Very different arrangements have been made by different writers on the symptoms manifested by the insane, and they have grouped them under somewhat opposite heads, guided in some instances by the most prominent symptoms, and in others, by the supposed seat of the malady. Before that attention had been paid to psychological affections, which they now happily have obtained, Cullen's nosology was regarded as clear and natural; and doubtless, it promised advantages over the systems of Sauvages, Vogel, and Linnæus. Cullen placed mental disorders in the class, Neuroses, and under the order, Vesaniæ, which included those disorders in which the judgment is impaired, without coma or pyrexia. These he referred to four great divisions, viz., amentia, melancholia, mania, and oneirodynia. Amentia, might be either congenital, senile, or acquired; melancholia exhibited itself in eight principal varieties, some involving hallucinations of a painful, others of a pleasurable nature; mania, he defined as a false judgment arising from perversions of the imagination, or from false recollection, and commonly producing disproportionate emotions (assuming the person to be awake and free from pyrexia and coma); oneirodynia, the last of Cullen's divisions, included, somnambulism and night-mare.