 To represent an intervention on a variable, we replace the equation for that variable with a new equation stating the value to which the variable is set. For example, if we intervene to set the level of flame at low, we would represent this by replacing the equation Flame = Gas level \(\times\) Igniter with Flame = 1. This creates a new causal structure in which Flame is an exogenous variable; graphically, we can think of the intervention as “breaking the arrows” pointing into Flame. The new system of equations can then be solved to discover what values the other variables would take as a result of the intervention. In the world described above, our intervention would produce the following set of equations: