Model validation constitutes a very important step in system dynamics methodology. Yet, both published and informal evidence indicates that there has been little effort in system dynamics community explicitly devoted to model validity and validation. Validation is a prolonged and complicated process, involving both formal/quantitative tools and informal/qualitative ones. This paper focuses on the formal aspects of validation and presents a taxonomy of various aspects and steps of formal model validation. First, there is a very brief discussion of the philosophical issues involved in model validation, followed by a flowchart that describes the logical sequence in which various validation activities must be carried out. The crucial nature of structure validity in system dynamics (causal‐descriptive) models is emphasized. Then examples are given of specific validity tests used in each of the three major stages of model validation: Structural tests, structure‐oriented behavior tests and behavior pattern tests. Also discussed is if and to what extent statistical significance tests can be used in model validation. Among the three validation stages, the special importance of structure‐oriented behavior tests is emphasized. These are strong behavior tests that can provide information on potential structure flaws. Since structure‐oriented behavior tests combine the strength of structural orientation with the advantage of being quantifiable, they seem to be the most promising direction for research on model validation.

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System dynamics models, as causal models, are much like scientific theories. Hence, in evaluating such models, we assume certain norms of scientific inquiry. Most critics hold that the system dynamics approach does not employ formal, objective, quantitative model validation tests. This article argues that this type of criticism presupposes the traditional logical empiricist philosophy of science, which assumes that knowledge is an objective representation of reality and that theory justification can be an objective, formal process. According to the more recent relativist philosophy of science, knowledge is relative to a given society, epoch, and scientific world view. Theory justification is therefore a semiformal, relative social process. We show that relativist philosophy is consistent with the system dynamics paradigm and discuss the practical implications of the two philosophies of science for system dynamics modelers and their critics. System Dynamics Review 6 (no. 2, Summer 1990): 148-166. ISSN 0883-7066. 0 1990 by the System Dynamics Society. EpistemologyRE& DESCARTES' RATIONALISM. The idea of developing a coherent theory of knowledge can be traced to Ren6 Descartes . Descartes believed that philosophy needed a new method, the deductive reasoning of mathematics, because the only truths that could be accepted without any doubt were ones revealed by this method. He claimed that purely deductive reasoning was possible because the ideas of such reasoning were innate, prior to all experience. He was a pure rationalist. In his famous Meditations on First Philosophy (1641), Descartes uses his "method of doubt" and deductive reasoning in order to find out what we can believe with certainty and what we can doubt. He concludes that the mind ("Thinking Self') exists with certainty

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