beginning that such situations could not be completely described; the idea was simply to give some statements about situations, and derive consequences from them. This is also different from the approach that is taken by the fluent calculus, where a state can be a collection of known facts, that is, a possibly incomplete description of the universe. In the original version of the situation calculus, fluents are not reified. In other words, conditions that can change are represented by predicates and not by functions. Actually, McCarthy and Hayes defined a fluent as a function that depends on the situation, but they then proceeded always using predicates to represent fluents. For example, the fact that it is raining at place in the situation is represented by the literal . In the 1986 version of the situation calculus by McCarthy, functional fluents are used. For example, the position of an object in the situation is represented by the value of , where is a function. Statements about such functions can be given using equality: means that the location of the object is the same in the two situations and . The execution of actions is represented by the function : the execution of the action in the situation is the situation . The effects of actions are expressed by formulae relating fluents in situation and fluents in situations . For example, that the action of opening the door results in the door being open if not locked is represented by: The predicates and represent the conditions of a door being locked and open, respectively. Since these condition may vary, they are represented by predicates with a situation argument. The formula says that if the door is not locked in a situation, then the door is open after executing the action of opening, this action being represented by the constant . These formulae are not sufficient to derive everything that is considered plausible. Indeed, fluents at different situations are only related if they are preconditions and effects of actions; if a fluent is not affected by an action, there is no way to deduce it did not change. For example, the formula above does not imply that follows from , which is what one would expect (the door is not made locked by opening it). In order for inertia to hold, formulae called frame axioms are needed. These formulae specify all non-effects of