  In this paper we review the notion of direct causal effect as introduced by
Pearl (2001). We show how it can be formulated without counterfactuals, using
intervention indicators instead. This allows to consider the natural direct
effect as a special case of sequential treatments discussed by Dawid and
Didelez (2005) which immediately yields conditions for identifiability as well
as a graphical way of checking identifiability. The results are contrasted with
the criteria given by Pearl (2001) and Robins (2003).
