Sigmund Freud (1856-1939) divided the mind into three conflicting internal tendencies: id, ego, and superego. The id, operating at the unconscious level, is governed by the pleasure principle and is unaware of the realities of the physical world. The ego, which derives its power from the id and cannot exist independently of it, coordinates the needs of id and the physical world. It perceives and manipulates the physical world in service of the id. Upon discovering a real object or event that will satisfy a need, the ego invests libidinal energy into thoughts about it, a process known as cathexis. However, the resolution may be unacceptable to the superego, resulting in guilt. Consequently, libidinal energy is expended to inhibit the association between the need and the object or event, a process known as anticathexis.