This paper, which in part serves as a common introduction to the two papers following in this issue, attempts to define the meaning of the "casual interpretability of a parameter" in a system of simultaneous linear relationships. It attempts, moreover, to expound a basis for interpreting the parameters of a nonrecursive or interdependent system casually. This is done in terms of an underlying causal chain system to which the interdependent system is either an approximation or a description of the equilibrium state.