 Consider the following case, adopted from Hesslow (1976): suppose that birth control pills (B) prevent pregnancy (P). However, they can also have the unintended side-effect of thrombosis (T). So we might be inclined to accept the type causal claim “birth control pills cause thrombosis”. But wait: another potential cause of thrombosis is pregnancy. And birth control pills inhibit pregnancy. The general structure of the case is as shown below.