It is an axiom in medicine that correct diagnosis is the indispensable preliminary to the rational and intelligent treatment of disease. This fundamental principle is universally recognised in dealing with bodily affections; but it is the primary object of this book to insist that _it is equally necessary to observe the same principle in the case of mental illness_. It may seem ironical to stress this elementary consideration, but it is notorious that accurate diagnosis is too often ignored in cases of incipient mental disturbance. It is idle to pretend that such a procedure is unnecessary, or to urge in extenuation of the failure to search for causes that many patients recover under the influence of nothing more than rest, quiet, and ample diet. Many mild cases of illness, whether bodily or mental, may and do recover even if undiagnosed or untreated. But on the other hand many mild cases get worse; and it is the primary duty of the physician correctly to diagnose the nature of the trouble and to give a prognosis—to decide whether the illness is mild or severe. Some of the most serious cases of incipient mental trouble are those of patients who do not seem to be really ill, and are easily overlooked by a visiting physician. They are quiet and inoffensive and display no obvious signs of the insidious processes that are at work in them. But all the time they may be, and often are, brooding over some grievance or moral conflict, worrying about their feelings, misinterpreting them and gradually systematising these misunderstandings until they become set as definite delusions or hallucinations. If, acting on the belief that it is bad to talk about a patient’s worries, the physician leaves such a man alone, he is clearly neglecting his obvious duty. For the whole trouble may be due to some trivial misunderstanding which he could easily correct. In the severer forms of mental disease, precise diagnosis is even more intimately related to treatment than in the case of bodily illness. For when a patient’s illness is recognised as some bodily affliction, such as pneumonia or appendicitis, certain general lines of treatment are laid down as soon as the appropriate label has been found for the complaint, though, in the case of the latter illness, there is added the further problem of whether or not surgical interference is indicated.