Qualitative Simulation: Then and Now

Qualitative reasoning about physical systems has become one of the most active and productive areas in AI in recent years. While there are many different kinds of qualitative reasoning, the central role is played by qualitative simulation: prediction of the possible behaviors consistent with incomplete knowledge of the structure of physical system. In the retrospective on my 1984 paper, “Commonsense reasoning about causality: deriving behavior from structure”, I describe the framework for qualitative reasoning that has motivated this work, and the applications that have come out of that framework. That paper includes the conjecture that the structural and behavioral representations for qualitative simulation could be rigorously shown to be abstractions of ordinary differential equations and their solutions. My 1986 paper, “Qualitative simulation”, established that conjecture and legitimized the term qualitative differential equation or QDE. It also presented the clear and efficient QSIM algorithm. In this retrospective, I describe aspects of the body of work on qualitative simulation that has developed from there.