=Medicine.= In the first place, there is a department of medicine which deals with nervous diseases, such as insanity, double personality, severe nervous shock, hallucination, etc. This entire aspect of medicine is wholly psychological. But psychology can be of service to the general practitioner both in the diagnosis and treatment of disease. A thorough psychological knowledge of human nature will assist a physician in diagnosis. Often the best way to find out what ails a patient's body is through the patient's mind, and the doctor must know how to get the truth from the patient's mind even in those cases in which the patient is actually trying to conceal the truth. A profound practical knowledge of human nature is necessary,--a knowledge which can be obtained only by long and careful technical study as well as practice and experience. Psychology can be of service in the treatment of disease. The physician must understand the peculiar mental characteristics of his patient in order to know how to deal with him. In some cases, hypnotism is a valuable aid in treatment, and in many cases, ordinary normal suggestion can be of considerable service. The state of mind of a sick person has much to do with his recovery. The physician must know this and must know how to induce the desired state of mind. Indeed, a patient's trouble is often imaginary, exists in the mind only; in such cases, the treatment should be wholly mental, _i.e._ through suggestion. Of course, the best physicians know these facts and make use of them in their practice, but preparation for this aspect of their work should be a regular part of their medical education. They should not be left to learn these facts from their practice as best they may, any more than they should be expected to learn their physiology and anatomy in this way. =Law.= The service of psychology to law can be very great, but owing to the necessary conservatism of the courts, it will be a long time before they will make much use of psychological knowledge. Perhaps the greatest service will be in determining the credibility of evidence. Psychology can now give the general principles in this matter. Witnesses go on the stand and swear to all sorts of things as to what they heard and saw and did, often months and even years previously. The expert clinical psychologist can tell the court the probability of such evidence being true. Experiments have shown that there is a large percentage of error in such evidence. The additional value that comes from the oath has been measured. The oath increases the liability of truth only a small percentage. Experiments have also shown that one's feeling of certainty is no guarantee of truth. Sometimes the point we feel surest about is the one farthest from the truth. In fact, feeling sure of a thing is no guarantee of truth.