  When the causal relationship between X and Y is specified by a structural
equation, the causal effect of X on Y is the expected rate of change of Y with
respect to changes in X, when all other variables are kept fixed. This causal
effect is not identifiable from the distribution of (X,Y). We give conditions
under which this causal effect is identified as the solution of an integral
equation based on the distributions of (X,Z) and (Y,Z), where Z is an
instrumental variable.
