 As this form of counterfactual suggests, the structural equations are to be read from right to left: the antecedent of the counterfactual states possible values of the variables X1 through to Xn and the consequent states the corresponding value of the endogenous variable Y. There is a counterfactual of this kind for every combination of possible values of the variables X1 through to Xn. It is important to note that a structural equation of this kind is therefore not, strictly speaking, an identity since there is a right-to-left asymmetry built into it. This asymmetry corresponds to the asymmetry of non-backtracking counterfactuals. For example, supposing that the actual situation is one in which neither Suzy nor Billy throws a rock so the bottle does not shatter, the non-backtracking counterfactual “If either Suzy or Billy had thrown a rock, the bottle would have shattered” is true. But the counterfactual “If the bottle had shattered, either Suzy or Billy would have thrown a rock” is false. 