REAL PHYSICS FOR REAL ENGINEERS: RESPONSE TO “PROLEGOMENA TO ANY FUTURE QUALITATIVE PHYSICS”

In “Prolegomena,” Sacks and Doyle (1992) cite the shortcomings of existing qualitative physics systems with regard to “expert” reasoning about physical systems and offer a pathway for future research in the field. While we do not entirely share in their criticism of the research done to date, me do believe they provide important insights concerning expert reasoning and offer useful advice for future research directions. The authors correctly point out many deficiencies of SPQR with regard to expert reasoning. Expert reasoning, however, was not the stated goal of SPQR systems. The authors are, in effect, questioning the broad goals and assumptions of qualitative physics. We believe that automating expert reasoning is indeed an important goal for qualitative physics and that Sacks and Doyle present a useful approach to this goal. In measuring the performance of existing qualitative physics systems relative to expert performance, Sacks and Doyle imply somewhat different goals for qualitative physics than those stated in the past. For instance, Forbus states: “The goal of qualitative physics is to capture both the common-sense knowledge of the person on the street and the tacit knowledge underlying the quantitative knowledge used by engineers and scientists” (Forbus 1990). The expert reasoning described by Sacks and Doyle is very different from the tacit or commonsense reasoning described by Forbus. Commonsense or naive physics is commonly characterized as extrapolating (sometimes incorrectly) from everyday experience to draw conclusions about device or system behavior. (A nice example is the contention by the friend of one of the reviewers that putting a blanket over the hood of a car on a cold winter’s night will make it easier to start the car the next morning. When asked why he thought it would work, he explained, “Well, blankets make things warmer don’t they? ”) In reviewing qualitative physics literature, however, we find that the research is increasingly representative of “expert” qualitative reasoning than of commonsense or naive reasoning. While it is unfair to criticize past work relative to unsought goals, it is clear that expert reasoning about physical systems is indeed a current goal of qualitative physics. Therefore the rationale behind rejecting traditional methods of physicists and engineers should be reexamined. We agree with Sacks and Doyle’s assertion that qualitative physics research has made some inappropriate assumptions regarding the needs and methods of experts. A perception exists that the “tacit knowledge” of engineers and scientists and the formalisms learned in school and career practice are somehow separable, and that only the capture of this “tacit knowledge” is suitable for qualitative physics. In contrast, we believe that with respect to physical systems a qualitariviry continuum exists from the naive or common sense reasoning to purely quantitative reasoning. To understand system behavior, experts do not immediately formulate very specific equations and crunch numbers. Scientists and engineers are taught to use a wide variety of qualitative methods. The phase-plane analysis example presented by Doyle and Sacks is one of many such methods used for frequency and stability analysis. Numerous additional expert strategies such as sequential linearization, monotonicity analysis, convexity analysis, perturbation methods, dimensional anal-