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.