A T FIRST glance, such a question as that posed in the title of this paper may seem absurd. The tendency would be to answer immediately: "Yes, of course", and to refer to the field of medicine, where it is quite apparent that therapeutic measures are determined by differential diagnosis. Thome writes in this connection: "It seems elemental that rational treatment cannot be planned and executed until an accurate diagnosis has been made" (P, p. 319).On the other hand, Rogers has just as positively stated that "diagnostic knowledge and skill is not necessary for good therapy" (7, p. 421).Before proceeding further it should be made clear that we are concerned with the so-called functional disorders in which psychotherapy is applicable. It is recognized that there are mental disorders of definite organic origin, involving neurological disease, physiological disturbances, toxic conditions, and traumatic injury. There are also certain mental disorders which are possibly organic in nature, on a constitutional or endocrinological basis, e.g., the so-called endogenous depressions. Diagnosis of organic factors is an important medical function in these cases (assisted often by the use of psychological tests), and obviously influences therapy. Such diagnosis is, however, often difficult, and sometimes inconclusive, and psychotherapy should not be denied pending decision as to the presence or absence of .organic pathology. Moreover, although psychotherapy may not be indicated for a purely organic disorder, there are often mixtures of organic and psychological components in which psychotherapy is beneficial. In addition, the presence of a purely organic condition may, and often does, 1 The author wishes to acknowledge the stimulation of the members of an informal seminar, including