Recall that a causal effect is defined as the difference between two potential outcomes. In this case, there are multiple causal effects because there are more than two potential outcomes. One is the causal effect of the drug on Joe when Mary receives treatment and is calculated, formula_13. Another is the causal effect on Joe when Mary does not receive treatment and is calculated formula_14. The third is the causal effect of Mary's treatment on Joe when Joe is not treated. This is calculated as formula_15. The treatment Mary receives has a greater causal effect on Joe than the treatment which Joe received has on Joe, and it is in the opposite direction.