Total causal effect

For binary exposure $A$, the total causal effect on an additive scale is $\mathbb{E}[Y(1)-Y(0)]$. With mediator potential outcome $M(a)$ and nested outcome $Y(a,m)$, consistency gives $Y(a)=Y(a,M(a))$.

One natural direct effect iswhich changes exposure while holding the mediator at its natural value under no exposure.

One natural indirect effect iswhich changes the mediator between its two natural values while holding exposure fixed.