 Some terminology: given a system of structural equations, the variables which appear on the left-hand-side of some equation are called the endogenous variables. The variables which only ever appear on the right-hand-side of the equations are called exogenous. The model does not tell us anything about how the values of the exogenous variables are determined; but it does tell us something about how the values of the endogenous variables are causally determined by the values of the other variables in the model.