 There are, however, well-known examples of systems that potentially show a misapplication of the faithfulness assumption. One such example, originating in Hesslow (1976), involves a contraceptive pill that can cause thrombosis while simultaneously lowering the chance of pregnancy, which can also cause thrombosis. As Cartwright (2001: 246) points out, given the right weight for these process, it is conceivable that the net effect of the pills on the frequency of thrombosis be zero. This is a case of “cancelling paths”, where the effect of two or more causal routes between a pair of variables cancels to achieve statistical independence. In a case such as this, since we can have independent knowledge of the separate causal mechanisms involved here, there are grounds for arguing that there really is a causal connection between the variables despite their statistical independence. Thus, it is certainly possible to imagine a scenario in which the faithfulness assumption could lead us astray. However, in defense of the general principle, an example such as this clearly contains what Wood and Spekkens refer to as fine tuning; the specific weights for these processes would need to match precisely to erase the statistical dependence, and such a balance would generally be thought as unstable (any change in background conditions, etc. would reveal the causal connection in the form of a statistical dependence).