A comprehensive survey of the relations between psychology and psychiatry. Freud is cited as the first to furnish causal psychological explanations of psychotic phenomena. 'The formulations of the psycho-analyst do enable us to interpret in psychological terms the conditions with which the psychiatrist has to deal.' The differing causal explanations of Jung and Adler also achieve some measure of success. Psychological attack has adhered strictly to the methods of science. It is open to doubt whether mechanistic scientific method will prove adequate to explain mental phenomena. The time for comprehensive generalization is still distant. At present we cannot afford to neglect any line of approach, physiological or psychological, which throws light on any facet of the complex problem of psychiatry.