 In other words, the intervention disrupts completely the relationship between \(X_i\) and its parents so that the value of \(X_i\) is determined entirely by the intervention. Furthermore, the intervention is surgical in the sense that no other causal relationships in the system are changed. Formally, this amounts to replacing the equation governing \(X_i\) with a new equation \(X_i = x_i\), substituting for this new value of \(X_i\) in all the equations in which \(X_i\) occurs but leaving the other equations themselves unaltered. In a graphical representation of causal relationships (see below), an intervention of this sort on a variable \(X_i\) breaks or removes all other arrows directed into \(X_i\), so that the value of \(X_i\) is now completely fixed by the intervention. Pearl’s assumption is that the other variables that change in value under this intervention will do so only if they are effects of \(X_i\).