Reflections stimulated by the comments of Shadish (2010) and West and Thoemmes (2010)

This article offers reflections on the development of the Rubin causal model (RCM), which were stimulated by the impressive discussions of the RCM and Campbell's superb contributions to the practical problems of drawing causal inferences written by Will Shadish (2010) and Steve West and Felix Thoemmes (2010). It is not a rejoinder in any real sense but more of a sequence of clarifications of parts of the RCM combined with some possibly interesting personal historical comments, which I do not think can be found elsewhere. Of particular interest in the technical content, I think, are the extended discussions of the stable unit treatment value assumption, the explication of the variety of definitions of causal estimands, and the discussion of the assignment mechanism.