 One worry with the weak requirement is that the counterfactuals in (1) fail to capture the modularity of the structural equations. Recall: it’s important to distinguish between the structural equations \(Y \coloneqq X\) and \(Z \coloneqq X + Y\) and the equations \(Y = X\) and \(Z=X+Y\). Modularity is not just the claim that, if we were to intervene to set X equal to x, the equations \(Y = X\) and \(Z=X+Y\) would continue to be true. It is the claim that, if we were to intervene to set X equal to x, the structural equations \(Y \coloneqq X\) and \(Z \coloneqq X + Y\) would continue to hold. This requires, at least, that the counterfactuals