The purpose of psychotherapy does not differ essentially from that of the treatment of sickness generally. It is to relieve distress and promote efficiency of mind while improving the patient's adaptation to the group in which he lives, to their mutual benefit. Before psychotherapy can be prescribed or undertaken, a diagnostic review is necessary of the relationship between the patient's psychophysical constitution and the stresses to which he has been and is being exposed. This relationship is a complex one which involves the effects of previous stresses and the way in which they have been surmounted or have left their mark. The introduction of a third complex entity, the method and personality of the therapist, renders the total situation too involved for detailed comprehension and logical analysis, especially if the therapy is to be of the causal-anamnestic or comprehensive kind which is not content simply to deal with symptoms and their immediate causes. In the presence of this degree of complexity—well beyond the range of mental encompassment by one mind at one time—the psychotherapist must walk in humility, for he can rarely feel justified in thinking that he knows exactly what is going on.