Thanatos

"..In classical Freudian psychoanalytic theory, the death drive ("Todestrieb") is the drive towards death, destruction and forgetfulness. It was first proposed by Sigmund Freud in Beyond the Pleasure Principle. The death drive opposes Eros, the tendency towards cohesion and unity. The death drive is sometimes referred to as "Thanatos" in post-Freudian thought, although this term has no basis in Freud's own work.'

When Freud worked with people with trauma (particularly the trauma experienced by soldiers returning from World War I), he observed that subjects often tended to repeat or re-enact these traumatic experiences, a phenomenon that Freud called repetition compulsion. This appeared to violate the pleasure principle, the drive of an individual to maximize his or her pleasure. Freud found this repetition of unpleasant events in the most ordinary of circumstances, even in children's play (such as the celebrated Fort/Da (Gone/There) game played by Freud's grandson, who would stage and re-stage the disappearance of his mother and even himself).

Freud's initial dichotomy between the reality principle (Ego) and the pleasure principle (Id) was unable to account for this phenomenon, as well as several other clinical phenomena, including primary masochism and depression. It was difficult to attribute such non-pleasurable activity to either the self-preserving ego or to the libidinal instincts solely focused on pleasure....

...To explain this discrepancy, Freud postulated the existence of a fundamental death drive that would counterbalance the tendency of beings to do only what they find pleasurable. Organisms, according to this idea, were driven to return to a pre-organic, inanimate state.

In doing so, Freud kept his earlier instinct theory almost intact, while omitting the property of reversal of content used to compensate for non-pleasure-principle behaviours of the sexual instincts, replacing it with a separate instinct of destruction and aggression not influenced by the pleasure principle.

Thus, for example, masochism is no longer the reversal of content of the sexual/self-preserving instincts, but rather the change of objects of sadism from external to internal, notably to the ego. Sadism is thus considered "a direct manifestation of the death instinct".http://en.wikipedia.org/wiki/Death_drive