From the psychological point of view demons are nothing other than intruders from the unconscious, spontaneous irruptions of unconscious complexes into the continuity of the conscious process. Complexes are comparable to demons which fitfully harass our thoughts and actions; hence in antiquity and the Middle Ages acute neurotic disturbances were conceived as possession. Thus, when the individual consistently takes his stand on one side, the unconscious ranges itself on the other and rebels. Hence, if the individuality fails to differentiate itself from the opposites, it becomes identical with them and is inwardly torn asunder, so that a state of agonizing disunion arises.

Psychological Types by Carl Gustav Jung