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A theory of a phenomenon is an explanation of it, showing how it follows as a conclusion from general propositions in a deductive system. With all its empirical achievements, the functional school never produced a theory that was also an explanation, since from its general propositions about the conditions of social equilibrium no definite conclusions could be drawn. When a serious effort is made, even by functionalists, to construct an explanatory theory, its general propositions turn out to be psychological-propositions about the behavior of men, not the equilibrium of societies.
George C. Homans (Tue,) studied this question.