Things That Change by Themselves

Some fluents (for instance, time) change even after events that are not assumed to affect them (such as the action wait). We propose a formal-ization of the "commonsense law of inertia" in the situation calculus that allows us to describe such fluents. 1 Introduction The frame problem in Artificial Intelligence is the problem of specifying formally "what doesn't change when an event occurs" in such a way that the formal system "be ready to accept descriptions of new kinds of events and new kinds of fluents whose values are in general not affected by events whose descriptions don't mention them" [McCarthy, 1987].p The assertion that the value of a fluent / does not change when an action a is performed in a situation s can be expressed by the equation (1) The question is how to express that this is true "in general ," with the exceptions corresponding to the cases when other axioms postulate a new value for /. The default principle according to which the values of fluents are presumed to remain unchanged is sometimes called the "commonsense law of inertia." Formalizing this principle is considered one of the central problems in formal nonmonotonic reasoning. The proposal of [McCarthy, 1986] was to restrict (1) to the triplesthat do not satisfy an "abnormality" predicate Circumscription is used to ensure that the extent of ab is minimal. Section 12. Several modifications that fix these problems have been found; for references and a critical discussion, see [Hanks and McDermott, 1987]. We use the terminology and notation of the situation calculus [McCarthy and Hayes, 1969]. One of the proposed solutions [Lifschitz, 1987] describes the effects of actions using the predicate causes. For instance, the effect of moving a block b to a location / can be expressed by the axiom The commonsense law of inertia can be postulated then in the following form: The predicate causes is circumscribed; as a result, the antecedent of (3) is "generally true," and (3) can serve as the desired weakening of (1). Axiom (3) asserts that the value of a fluent does not change after an action is performedthat action causes the fluent to take on some In this paper we argue that, in many domains, this interpretation of "commonsense interia" is inappropriate. There are fluents that may change no matter what action is performed. The simplest example is the integer-valued fluent time, characterized by the …