Narrow Views, Old Tasks, and New Beginnings

Abstract

One can respond to "Prolegomena to any Future Qualitative Physics" at several levels. As practitioners in the field, it is easy to become incensed and respond to every minute misrepresentation and error. This would unfortunately miss the forest for the trees. Unfortunate, because there is a much simpler story to be told here. Sacks and Doyle make two high-level claims as the basic theme of their paper. First, they state that the field of qualitative reasoning is roughly equivalent to SPQR. In other words, the principal goal of the qualitative reasoning enterprise is to (1) produce abstract state transition diagrams of system behavior (i.e., simulations) which are (2) solely for the purpose of predicting behavior. Second, in reasoning about such systems' behavior, "experts" are predominantly concerned with their asymptotic, dynamical properties.As stated, we view both points as incorrect, misleading, and quite surprising coming from Sacks, who, as a frequent attendee of the qualitative reasoning workshops, should have a better understanding of the field. However, there is another, less controversial interpretation of the paper which we obtained from personal conversations with Sacks at the most recent workshop. First, their argument was intended to address work on the SPQR subset of the field rather than suggest that all of qualitative reasoning is SPQR. Second, their reference to the concerns of an expert was refemng to an expert academic dynamicist, and was a suggestion about how to compare the relative merits of SPQR and dynamical analysis.Given this interpretation shift, Sacks and Doyle's paper raises an interesting, but much narrower, technical issue related to acausal simulation, a small portion of the work going on in the field of qualitative reasoning. One argument in favor of qualitative simulation (SPQR) is that qualitative differential equations enable reasoning about entire classes of functions and is thus a more efficient way to perform certain tasks, such as the preliminary stages of design. Alternatively, Sacks and Doyle argue that it is better to pick a specific differential equation, analyze its solutions using conventional techniques, and if these are undesirable (for the task, say diagnosis or design) to pick another one. We believe that neither has demonstrated clear superiority for the kinds of tasks qualitative simulation is intended to address and we think it is perfectly reasonable that researchers in qualitative simulation explore and evaluate both approaches.Rather than focus on that debate in our response, we instead address the broader issues that have arisen while discussing this paper. We try to clarify what we see as the goals of the field and describe some of the major research efforts. 'We were told that they could not make these clarifying changes because some people had already invested significant effort responding to the original paper.