The study of conditioned reflexes becomes fully useful for psychology only with the introduction into it of the concept

of the pleasure principle. It is no undue assumption that all behavior of animals and human beings springs from the tendency to achieve pleasure and to avoid pain. That assumption is a key for translating the objective findings of the physiologist into subjective psychological language and vice versa. The coöperation of both groups of investigators will lead to a new organon of psychology, as the coöperation of experimental physics and mathematical speculation led to the all-important development of modern theoretical physics.