Freud describes the human mind as interaction of id, ego, super-ego. The id represents a constant in the personality as it the ego and the id sigmund freud pdf always present.
The id is governed by the ‘pleasure principle’. Early in the development of his theory Freud saw sexual energy as the only source of energy for the id. Freud felt it necessary to add another instinct to the id. So, he proposed thanatos, the death instinct. Obviously, the rest of the personality would have somehow to deal with these two instincts.