 If the system of equations is modular, then it is at least in principle possible to disrupt one of the equations without affecting any of the others. Suppose that happens, and we disrupt the equation \(A\coloneqq B \wedge \neg C\) without affecting any of the other equations, for instance. Suppose further that we do this in such a way as to determine the value of \(A\), or to determine a probability distribution over the values of \(A\). Then, we have performed an intervention on the variable \(A\). Note that this notion of an intervention is relative to a system of equations. Whether some way of disrupting an equation and directly setting the value of the variable on its left-hand-side counts as an intervention or not will vary from causal model to causal model. In general, an intervention on an endogenous variable, \(V\) (relative to some model) is some way of making it so that \(V\)’s structural equation (the equation with \(V\) on the left-hand-side) no longer holds, even though every other structural equation in the model continues to hold, and directly setting the value of \(V\), or directly determining a probability distribution over the values of \(V\).