Authors who have treated on the subject of _Insanity_ have anxiously attempted to frame a definition of the malady; and, by compressing into a short sentence its prominent and distinguishing phenomena, to establish a fixed and essential character. In this attempt each author has fundamentally differed, and to enumerate their plans would be only to expose their failures; the truth is, that the varied and mutable phenomena of insanity will ever mock the grasp of the nosologist; instead therefore of endeavouring to discover an infallible definition, it will be of much greater importance to investigate the circumstances which should guide the medical witness in a decision that may annul a man’s dominion over property, involve his contracts and other acts which otherwise would be binding, and take away his responsibility for crimes. Modern authors, according to the system of the Grecian writers, have generally divided mental derangement into two classes—_Mania_[523] and _Melancholia_;[524] the former being distinguished by a state of extraordinary excitement, the latter by great depression; although they are frequently convertible affections. _Mania_ may be said to be a state of mental alienation, accompanied by an unusual ferocity in language and deportment, and by a comparative insensibility to ordinary stimuli. _Melancholia_ is a form of insanity which is always attended with some seemingly groundless, but very anxious fear, by which the person is plunged into a gloomy and desponding state, that not unfrequently leads to the commission of suicide.