Some investigators have posited a domain-independent representation for knowledge called the problem space. Problem spaces, such as the type introduced by STRIPS, are commonly composed of a set of goals, a state or set of states, and a set of valid operators which contain the constraints under which the operator can be applied. The top-level goal is the problem originally posed to the agent. New goals are generated when the agent does not know how to apply any of its available operators rationally to move closer to its goal. The state consists of a set of literals that describe the knowledge of the agent and the present model of the world.