One of the major contributions of the American psychiatrist Harry Stack Sullivan (1892-1949) was the elimination of the concept of the mind from his system of psychiatric thought. He based all his concepts of personality development, and the nature of those psychiatric illnesses which are caused by emotional factors, on interpersonal relationships. Interpersonal relationships, in contrast to the functions of the mind, can be directly observed, and any principles based on interpersonal relationships can be submitted to experiments which can be proved or disproved by psychiatric investigators. The works of Sullivan thus furnish a path by which psychology and psychiatry can in time develop into truly scientific specialties.