Theory gains power as it moves toward the analysis of the systems. The logical requirements include definition of mutually interacting, functionally related variables, each with a single dimension. The system must be closed, but will be opened and reclosed for analysis. Systems need not be in either stable or unstable, static or moving equilibrium. To gain empirical relevancy and analytical power analysis must include systems not in equilibrium. In the study of societies, these logical requirements have been imperfectly understood.
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Everett E. Hagen (1961) studied this question.