From a purely theoretical standpoint, the educated physician—one whose motto is _Esse, non videri_—after combating a disease in vain, should not feel content to remain in ignorance of its real nature, so far as discoverable by anatomical changes, capable of being recognized after death, even had he no prospect of adding thereby one jot to his practical acquaintance with disease or to his power to combat it; yet his scientific conscience (if we may be allowed the expression) ought not to rest satisfied until, in all doubtful cases, his _ante-mortem_ diagnosis be confirmed or overthrown, and his conception of the case completed in all its details by a _post-mortem_ examination. Besides this purely individual scientific interest, there are weightier practical reasons for an acquaintance with this branch of medical science by the physician as practitioner. Among the almost innumerable questions upon medical, theological, and miscellaneous subjects which the American public feels at liberty to propound to its medical advisers, none are of more frequent occurrence and none are more justifiable than the two: “What is the matter with the patient?” and “Will he, or can he, recover?” and to none is an answer more imperatively demanded. The public very naturally, and with reason, requires on the part of a physician the ability to make a diagnosis and a prognosis. It will not be satisfied with being told that the name of the disease is of no importance, that the doctor only wants to hear the symptoms; that he does not cure _diseases_, but removes the symptoms of disease, &c. Only an exceedingly well-trained public will accept these truisms as an equivalent for diagnostic skill. Hence, it is the physician’s interest, as well as his duty, as we shall see, to seek, in all cases, to make a diagnosis, no matter how difficult the task may prove to be. The question how far his treatment will be modified by his diagnosis is a question of therapeutics, and does not belong here; but certain it is, that a mere combatal of the symptoms as isolated phenomena cannot be regarded as fulfilling the whole duty of a conscientious physician. The ability to make a diagnosis, and consequently prognosis, depends upon a knowledge of pathology, with a knowledge of symptoms as signs of pathological states and changes; and as such they must be critically examined and their true import discovered, if possible. Thus treated, we arrive, by various processes of reasoning, at a diagnosis, under which the symptoms fall into their natural order of importance, and we run but little risk of contending with remote subjective symptoms (of great importance in differential therapeutics) to the neglect of more important, though perhaps less prominent ones.