the value of a particular fluent can be changed. The effect axioms affecting the value of fluent can be written in generalised form as a positive and a negative effect axiom: The formula describes the conditions under which action in situation makes the fluent become true in the successor situation . Likewise, describes the conditions under which performing action in situation makes fluent false in the successor situation. If this pair of axioms describe all the ways in which fluent can change value, they can be rewritten as a single axiom: In words, this formula states: "given that it is possible to perform action in situation , the fluent would be true in the resulting situation if and only if performing in would make it true, or it is true in situation and performing in would not make it false." By way of example, the value of the fluent introduced above is given by the following successor state axiom: States The properties of the initial or any other situation can be specified by simply stating them as formulae. For example, a fact about the initial state is formalized by making assertions about (which is not a state, but a situation). The following statements model that initially, the robot carries nothing, is at location , and there are no broken objects: Foundational axioms The foundational axioms of the situation calculus formalize the idea that situations are histories by having . They also include other properties such as the second order induction on situations. Regression Regression is a mechanism for proving consequences in the situation calculus. It is based on expressing a formula containing the situation in terms of a formula containing the action and the situation , but not the situation . By iterating this procedure, one can end up with an equivalent formula containing only the initial situation . Proving consequences is supposedly simpler from this formula than from the original one. GOLOG GOLOG is a logic programming language based on the situation calculus. The original version of the situation calculus The main difference between the original situation calculus by McCarthy and Hayes and the one in use today is the interpretation of situations. In the modern version of the situational calculus, a situation is a sequence of actions. Originally, situations were defined as "the complete state of the universe at an instant of time". It was clear from the