Instincts: The Propelling Force of Personality Freud interpreted the functioning of the human organism in physiological terms. From his medical training he knew that the human body operates by creating and expending a kind of physical energy. Food is transformed in the body into a form of energy that is used to fuel such functions as breathing. blood circulation, and muscular and glandular activity The mind also performs certain functions. It perceives the external world; it thinks, imagines, remembers. Using the analogy with the body, Freud assumed that the mind also carries out its functions through the use of energy--psychic energy, which differs in form but not in kind from the body's physical energy. He further assumed, based on the principle of the conservation of energy, that the body energy could be transformed into psychic energy, and vice versa. The energy of the body therefore influences the mind. The link between these two forms of energy, the frontier between the psychic and the somatic, lies in Freud's concept of instinct. Briefly defined, an instinct is the representation in the mind of stimuli that originate within the body. The instinct became Freud's basic element or unit of the personality. It is the motivating. propelling force of personality that not only drives behavior but also determines its direction. Freud's term in German for this concept is Trieb, which is best translated as a driving force or an impulse (Bettelheim, 1984). Instincts are a form of energy--trans- formed physiological energy -that serves to connect or bridge the body's needs to the mind's wishes.