  It is very natural to suppose that inertia holds by default; variables are unchanged by the performance of an action unless there is a special reason to think that they will change. This suggests that nonmonotonic temporal formalisms should provide an appropriate foundation for reasoning about action and change. So attempts to formalize the reasoning needed in planning also created a need for nonmonotonic logics. One of the earliest attempts to formalize nonmonotonic reasoning,  Sandewall 1972, addresses the frame problem. Inertial defaults are an especially important and instructive case study; no more will be said about them here, since they are discussed in detail in  Section 4.4,  below.