No class of diseases with which man is afflicted are so various in their manifestations as those known under the general term of insanity. No diseases present such an infinite variety of light and shade belonging to their own nature, or to their intermixture with other maladies, or to the influence of temperament, of individual peculiarities of habit, or of social position; and therefore the diagnosis of no other class of diseases taxes nearly so much the ingenuity and the patience of the physician. The Diagnosis of almost all other diseases depends principally upon weighing the evidence afforded by physical signs and symptoms, upon evidence addressed to the senses; but in mental disease, it is for the most part dependent upon evidence which is cognizable by the intellect alone, and upon data which the senses furnish to us only at second hand. The physician is compelled to bring to this investigation, not only a knowledge of those functions which are subservient to the vegetative and animal life of the individual, but also a clear and analytical conception of those which collectively constitute mind. He must not only be a physician, but a metaphysician; not indeed in the almost opprobrious sense of the term, but in that better sense which designates a lover of truth, seeking to ascertain not the essence of mind or any other unattainable abstraction, but the laws of mind which are as regular as any other natural laws, and the knowledge of which offers to philosophy a wholesome and legitimate object of research.