Comment on Hausman & Woodward on the causal Markov condition

Hausman & Woodward present an argument for the Causal Markov Condition (CMC) on the basis of a principle they dub 'modularity' ([1999, 2004]). I show that the conclusion of their argument is not in fact the CMC but a substantially weaker proposition. In addition, I show that their argument is invalid and trace this invalidity to two features of modularity, namely, that it is stated in terms of pairwise independence and 'arrow-breaking' interventions. Hausman & Woodward's argument can be rendered valid through a reformulation of modularity, but it is doubtful that the argument so revised provides any substantially new insight regarding the basis of the CMC.