MAD is therefore not a complex idea, as has been supposed, but a complex term for all the forms and varieties of this disease. Our language has been enriched with other terms expressive of this affection, all of which have a precise meaning. Delirium, which we have borrowed from the latin, merely means, _out of the track_, de lira, so that a delirious person, one who starts out of the track regularly pursued, becomes compared to the same deviation in the process of ploughing. _Crazy_, we have borrowed from the French _ecrase_, crushed, broken: we still use the same meaning, and say that such a person is crack'd. Insane, deranged, or disarranged,[1] melancholic, out of one's wits, lunatic, phrenitic, or as we have corrupted it, frantick, require no explanation. _Beside one's self_ most probably originated from the belief of possession by a devil, or evil spirit. The importance of investigating the original meaning of words must be evident when it is considered that the law of this country impowers persons of the medical profession to confine and discipline those to whom the term mad or lunatic can fairly be applied. Instead of endeavouring to discover an infallible definition of madness, which I believe will be found impossible, as it is an attempt to comprise, in a few words, the wide range and mutable character of this Proteus disorder: much more advantage would be obtained if the circumstances could be precisely defined under which it is justifiable to deprive a human being of his liberty. Another impediment to an accurate definition of madness, arises from the various hypotheses, which have been entertained concerning the powers and operations of the human mind: and likewise from the looseness and unsettled state of the terms by which it is to be defined. Before treating of the intellect in a deranged state, it will perhaps be expected that some system of the human mind, in its perfect and healthy condition, should be laid down. It will be supposed necessary to establish in what sanity of intellect consists, and to mark distinctly some fixed point, the aberrations from which are to constitute disease. To have a thorough knowledge of the nature, extent, and rectitude of the human faculties, is particularly incumbent on him who undertakes to write of them in their distempered state; and, in a legal point of view, it is most important that the medical practitioner should be enabled to establish the state of the patient's case, as a departure from that which _is_ reason.