 This definition generalises the informal idea sketched in the example of Suzy and Billy. &lt;ST, SH, BS&gt; is an active causal route because when we hold BH fixed at its actual value (Billy’s rock doesn’t hit the bottle), BS counterfactually depends on ST. By contrast, the route &lt;BT, BH, BS&gt; is not active because when we hold SH fixed at its actual value (Suzy’s rock does hit the bottle), BS does not counterfactually depend on BT.