In the original formulation of the situation calculus, the initial situation, later denoted by formula_7, is not explicitly identified. The initial situation is not needed if situations are taken to be descriptions of the world. For example, to represent the scenario in which the door was closed but not locked and the action of opening it is performed is formalized by taking a constant formula_27 to mean the initial situation and making statements about it (e.g., formula_95). That the door is open after the change is reflected by formula formula_100 being entailed. The initial situation is instead necessary if, like in the modern situation calculus, a situation is taken to be a history of actions, as the initial situation represents the empty sequence of actions.