2. The second diagnosis, that of the location of the lesions, is to be arrived at by the application of the observer's knowledge of the anatomy and physiology of the nervous system; and therefore it is here that special training is of the greatest advantage. The anatomy and physiology of use in this connection are not the bare sciences as taught in ordinary text-books, but a higher sort of knowledge, corrected and extended by the teachings of pathology and pathological anatomy. The physician must be well versed in the recent revelations of experimentation and of autopsies bearing upon the architecture and functions of the central nervous system, and should be able to apply this knowledge deductively to the case in hand. In this manner the now abundant material grouped under the term Localization of Cerebral and Spinal Diseases (vide the next article) can be made of the greatest utility in every-day practice. The solution of the problem of localization of the lesion is much simplified if it be first accurately determined whether the lesion is peripheral, spinal, or cerebral. 3. The final diagnosis is with respect to the nature of the lesion. While the second diagnosis may be said to possess the greater scientific interest, this one must be admitted to possess paramount practical importance, as from it we derive the indications for rational treatment and the data of prognosis. The third diagnosis is to be made by the application of the observer's knowledge of general pathology and etiology; hence it is in this field that the best-trained physician succeeds—where the experienced practitioner may sometimes eclipse the brilliant specialist. The truth of this is maintained by those who hold, as we do, that it is unwise to embark in specialism without having had good hospital advantages and extensive general practice. {63} In order to arrive at the diagnosis of the nature of the lesion we must consider the family history, trace out predispositions, study the various causes of disease to which the patient has been exposed, and by a thorough examination of the various functions and the objective condition of the patient ascertain what pathological processes are active in him. Often the clue to the diagnosis is found in signs afforded by non-nervous organs, as nervous syphilis by nodes and cutaneous cicatrices, cerebral hemorrhage by renal disease and increased arterial tension, cerebral tuberculosis by pulmonary lesions, etc. A purely empirical form of knowledge of some utility in proving the pathological diagnosis is that of the relative frequency of certain lesions in the two sexes, at different ages, in various professions, etc.—a statistical knowledge which is to be applied deductively to the case under study.