  Evaluation of counterfactual queries (e.g., "If A were true, would C have
been true?") is important to fault diagnosis, planning, determination of
liability, and policy analysis. We present a method of revaluating
counterfactuals when the underlying causal model is represented by structural
models - a nonlinear generalization of the simultaneous equations models
commonly used in econometrics and social sciences. This new method provides a
coherent means for evaluating policies involving the control of variables
which, prior to enacting the policy were influenced by other variables in the
system.
