It will be worth being explicit about the way the causal closure principle supports physicalism. First we assume that mental causes (biological, social, …) have physical effects. Then the causal closure principle tells us that those physical effects have physical causes. So, in order to avoid an unacceptable proliferation of causes for those physical effects (no “systematic overdetermination”), we need to conclude that the mental (biological, social, …) causes of those effects are not ontologically separate from their physical causes.