the ego and the id to that between a charioteer and his horses: the horses provide the energy and drive, while the charioteer provides direction. Life and death drives Freud believed that the human psyche is subject to two conflicting drives: the life drive or libido and the death drive. The life drive was also termed "Eros" and the death drive "Thanatos", although Freud did not use the latter term; "Thanatos" was introduced in this context by Paul Federn. Freud hypothesized that libido is a form of mental energy with which processes, structures, and object-representations are invested. In Beyond the Pleasure Principle (1920), Freud inferred the existence of a death drive. Its premise was a regulatory principle that has been described as "the principle of psychic inertia", "the Nirvana principle", and "the conservatism of instinct". Its background was Freud's earlier Project for a Scientific Psychology, where he had defined the principle governing the mental apparatus as its tendency to divest itself of quantity or to reduce tension to zero. Freud had been obliged to abandon that definition, since it proved adequate only to the most rudimentary kinds of mental functioning, and replaced the idea that the apparatus tends toward a level of zero tension with the idea that it tends toward a minimum level of tension. Freud in effect readopted the original definition in Beyond the Pleasure Principle, this time applying it to a different principle. He asserted that on certain occasions the mind acts as though it could eliminate tension, or in effect to reduce itself to a state of extinction; his key evidence for this was the existence of the compulsion to repeat. Examples of such repetition included the dream life of traumatic neurotics and children's play. In the phenomenon of repetition, Freud saw a psychic trend to work over earlier impressions, to master them and derive pleasure from them, a trend that was before the pleasure principle but not opposed to it. In addition to that trend, there was also a principle at work that was opposed to, and thus "beyond" the pleasure principle. If repetition is a necessary element in the binding of energy or adaptation, when carried to inordinate lengths it becomes a means of abandoning adaptations and reinstating earlier or less evolved psychic positions. By combining this idea with the hypothesis that all repetition is a form of discharge, Freud concluded that the compulsion