 It is part of the definition of a structural model that the exogenous variables are probabilistically independent of one another (e.g., Pearl 2000: 44). From this together with the assumption that a causal model is complete one can derive the causal Markov condition, which is a generalized common cause condition and states that for every variable X in V, X is probabilistically independent of the variables in the set \((V - \textrm{Descendants}(X))\) conditional on the parents of X.