Sacks and Doyle (1992) point out the absence of mathematics from SPQR (simulation of processes by qualitative reasoning). Undoubtedly QR (qualitative reasoning) at large should embody knowledge about linearity, dynamical systems theory, and numerical analysis. There already exists software, in the form of numerical algorithms written mostly in C or FORTRAN, automating mathematical techniques; and software which acts as an intelligent coordinating interface among numerical modules, as well as between a user and the numerical modules (e.g., Konar et al. 1990), is a step further in this direction.What is not automated is the formulation of real scientific and engineering problems in a solvable mathematical form; i.e., the reasoning of the expert scientists or engineers who are users of existing sophisticated mathematical software. Expert reasoning requires skills which are still informal in the experts' minds and need to be addressed by QR. These are largely domain-and task-specific skills; while some unifying formalisms will undoubtedly emerge, we may be better off looking for them only after we tackle several specific subdomains.Automated construction of models is one of the important issues, and there has been a lot of quiet work in that direction (e.g., Stephanopoulos et al. 1987). Experts formulate, analyze, and revise specific equations (Sacks and Doyle 1992); what QR and A1 should focus on is not the mathematical side of using equations but the model construction and revision process which uses (a) information about the physical system; (b) the structure and results of previously tried models, analyzed and compared; and (c) information about the ultimate applicarion of the model which determines the goals of the modeling effort, because expert assumptions and simplifications are always context dependent.The context dependence of modeling is one of the most important characteristics of expert reasoning. It has not been addressed by SPQR, but it also receives little attention in Sacks and Doyle (1992). The expert makes just the right assumptions, drops what is unimportant or negligible in each particular portion of the model and for the particular system and task at hand, and comes up with a model ofjust the right complexity.The models and values we use in practice are never accurate. Variables never assume an exact value and they never become exactly equal to each other. To use the -, 0, and + values of SPQR, we have to decide how small a value is considered qualitatively zero, and we similarly have to determine when two values are considered equal. Order-ofmagnitude reasoning systems, such as O[M] (Mavrovouniotis and Stephanopoulos 1987, 1988;Mavrovouniotis et al. 1989), FOG (Raiman 1986;Dague et al. 1987), and other systems (Dubois and Prade 1989), aimed to address, in part, the issue of distinguishing between negligible and important parameters. These systems focused on relationships between parameters (which is a better approach than direct reference to "small" and "large" values), but they did not addre...