  Natural direct and indirect effects decompose the effect of a treatment into
the part that is mediated by a covariate (the mediator) and the part that is
not. Their definitions rely on the concept of outcomes under treatment with the
mediator "set" to its value without treatment. Typically, the mechanism through
which the mediator is set to this value is left unspecified, and in many
applications it may be challenging to fix the mediator to particular values for
each unit or individual. Moreover, how one sets the mediator may affect the
distribution of the outcome. This article introduces "organic" direct and
indirect effects, which can be defined and estimated without relying on setting
the mediator to specific values. Organic direct and indirect effects can be
applied for example to estimate how much of the effect of some treatments for
HIV/AIDS on mother-to-child transmission of HIV-infection is mediated by the
effect of the treatment on the HIV viral load in the blood of the mother.
