In his later work, notably in "The Ego and the Id" (1923), a second topography is introduced comprising id, ego and super-ego, which is superimposed on the first without replacing it. In this later formulation of the concept of the unconscious the id comprises a reservoir of instincts or drives a portion of them being hereditary or innate, a portion repressed or acquired. As such, from the economic perspective, the id is the prime source of psychical energy and from the dynamic perspective it conflicts with the ego and the super-ego which, genetically speaking, are diversifications of the id.