Woodward’s (2001, 2002, 2011, 2014) counterfactual definition of a mechanism (which is indirectly specified via an account of mechanistic models), as well as a descendant elaborated by Menzies (2012), require that models of mechanisms be modular. This means, roughly, that it should be physically possible to intervene on a putative cause variable in a mechanism without disrupting the functional relationships among the other variables in the mechanism. In terms of structural equation models in particular, this means that one should be able to replace the right-hand side of an equation in the model with a particular value (i.e., set the left-hand variable to a value) without needing to change any of the other equations in the model. This is intended to formally capture the sense in which mechanism is composed of separable, interacting parts. For arguments in favor of a modularity condition on mechanistic models see Menzies (2012).