 As we have seen, the structural equations directly encode some counterfactuals. However, some counterfactuals that are not directly encoded can be derived from them. Consider, for example, the counterfactual “If Suzy’s rock had not hit the bottle, it would still have shattered”. As a matter of fact, Suzy’s rock did hit the bottle. But we can determine what would have happened if it hadn’t done so, by replacing the structural equation for the endogenous variable SH with the equation SH = 0, keeping all the other equations unchanged. So, instead of having its value determined in the ordinary way by the variable ST, the value of SH is set ‘miraculously’. Pearl describes this as a ‘surgical intervention’ that changes the value of the variable. In terms of its graphical representation, this amounts to wiping out the arrow from the variable ST to the variable SH and treating SH as if it were an exogenous variable. After this operation, the value of the variable BS can be computed and shown to be equal to 1: given that Billy had thrown his rock, his rock would have hit the bottle and shattered it. So this particular counterfactual is true. This procedure for evaluating counterfactuals has direct affinities with Lewis’s non-backtracking interpretation of counterfactuals: the surgical intervention that sets the variable SH at its hypothetical value but keeps all other equations unchanged is similar in its effects to Lewis’s small miracle that realises the counterfactual antecedent but preserves the past.