A S in other sciences, the boundaries between fact and theory in psychoanalysis cannot always be defined absolutely. Most of what we have so far surveyed deals with facts, with direct observations or immediate inferences from them. But the need to comprehend these facts as fully as possible has led to a systematic theory of the relationships of feeling, thought, and behavior. The most basic has been the theory of instincts, a hypothesis which is necessary for the coherent and dynamic integration of the psychological data with each other and with the data of biology.